Introduction: In dairy cattle enterprise, mastitis is a common and economically important disease leading to milk loss, therapeutic cost, changing of milk quality, reproductive problems, animal death, and early culling. A wide range of cow-level and herd-level factors can influence on the number of somatic cell count (SCC) including udder infection, lactation stage, age, breed, shortterm of days in milk, season, stress and level of production. This research aimed to estimate the effects of SCC on daily milk yield (DMY) at different lactation stages and months of production of first-parity cows (progeny of internal and external sperms). Variation of DMY in terms of Holstein gene inheritance and first calving age were special objectives. Material and methods: The data used in this research were provided by the Animal Breeding Centre, Karaj, Iran. Data set comprised a total number of 543, 884 test day records collected from 64, 634 first-parity dairy cows (progeny of 1, 899 sires and 56, 995 dams) distributed in 606 herds and calved between 2009-2013 (in ten provinces over the country). The trait under consideration (dependent variable) was DMY which is actually a quantitative variable with continuous distribution and measured based on ratio scale. The records were analysed using a linear fixed model. In the model, the effects of province, herd nested within province, year of calving, stage of lactation, sperm type of the sire, two-way interaction between stage of lactation and sperm type, as well as linear covariable of SCC within the two-way interaction, Holstein gene inheritance, and first calving age were taken into consideration. The model was fitted on the test day records of the cows for individual months of production using general linear model procedure of SAS software (Version 9. 4) and the effect of SCC was estimated as the partial regression coefficient in 30 combined groups resulted from two-way interaction between stage of lactation and sperm type. Results and discussion: Most of the partial regression coefficients were statistically significant (P<0. 05) and the negative sign obtained for the estimates indicates that DMY decreases as the number of SCC increases and that any regular trend was not detected for milk loss during different lactation stages and production months. Mean number of SCC ranged from 152, 000 (in SeptemberOctober) to 173, 000 (May-June). In terms of season of production, mean SCC in spring, summer, autumn and winter was 168, 000, 164, 000, 155, 000, and 160, 000, respectively. Average DMY for these seasons was 33. 83, 32. 62, 32. 83, and 33. 95 kg, respectively. Mean SCC ranged during different lactation stages so that for the first, second and third stages it was 185, 000, 147, 000 and 142, 000, respectively and after that an increasing trend was observed from the fourth (149, 000) toward the end (188, 000) of the lactation. Mean loss of DMY for the cows resulted from American sperms were 126, 121, 142, and 147 g in spring, summer, autumn, and winter, respectively. Maximum DMY loss for the cows resulted from Iranian, Canadian and American sperms were found to be in April-stage 10 (0. 2430 kg), April-stage 10 (0. 3853 kg), and March-stage 9 (0. 2305 kg), respectively. Minimum DMY loss for the corresponding cows were found in January-stage 1 (0. 0042 kg), November-stage 9 (0. 0015 kg), and August-stage 4 (0. 0447 kg), respectively. The findings also revealed that DMY significantly (P<0. 05) increased as the Holstein gene inheritance and age at first calving rose as one unit. Regarding Holstein gene inheritance, maximum and minimum increase of DMY were found in December-January (37. 6 g) and September-October (20. 4 g), respectively. For the age of cow at first calving, minimum and maximum corresponding figures were observed in July-August (232. 4 g) and June-July (215. 7 g), respectively. Conclusion: Daily milk yield of Iranian, American and Canadian sperms’ progeny decreases when somatic cell count of their milk increases and that it has no regular pattern at different lactation stages as well as different production months. Holstein gene inheritance and first calving age had positive effect on daily milk yield suggesting that these effects are needed to be taken into account as phenotypic and GENETIC analyses are carried out. Yearly milk production of daughters of the American sperms are apparently more suffered as compared to the other groups as a result of increasing SCC in DMY.