Resistance of Streptococcus pneumoniae against penicillin is considered to be of greatimportance. While low-resistant strains could be treated by penicillin, treatment of highly resistant strains is very difficult and needs broad-spectrum antibiotics. This study was performed in Imam Khomeini Medical Center, Tehran, Iran, from 1999 to 2001 to evaluate pneumococcal resistance against penicillin and some other antibiotics. Specimens were collected from different hospitals. Samples were cultured and resistance of S. pneumoniae against selected antibiotics was determined. The main aim of this study was to measure minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC), using serial dilution method. Disc diffusion method was also performed to be compared with the main procedures (MIC and MBC), on 66 strains obtained from 100 clinical specimens. Five different antibiotics (penicillin, cefazolin, ampicillin, amoxicillin and vancomycin) were employed in this study. In the case of penicillin, 47 sensitive strains and 19 highly resistant strains were obtained. No intermediate or low-resistant strain was found. The frequency of resistant strains against other antibiotics was found to be 10.6%, 7.5%, 18.1% and 0%, respectively. All strains were sensitive to vancomycin except for a low resistant one. Care should be taken to choose a suitable drug when being faced with a resistant strain. Vancomycin can be used with confidence to cure infections induced by penicilin-resistant S. pneumoniae.