SummaryThe in vitro bacteriostatic and bactericidal activity of eight antimicrobial agents has been evaluated on 107 pneumococci. Erythromycin and clindamycin were the most active compounds. Two strains were considered as relatively resistant to penicillin G. (MIC>0.16/ig/ml). Tetracycline and doxycycline showed the highest proportion of resistance while for chloramphenicol and cotrimoxazole the resistance rate was rather low. Chloramphenicol resistant strains were also resistant to tetracycline. The disk diffusion test with penicillin G appeared not to be a reliable test for detecting resistant pneumococci. Furthermore there was a bad correlation between MIC’s and zone diameters. In view of these findings we propose to do a routine MIC determination for aall pneumococci isolated from important clinical specimens.