In the submitted paper the authors give an account of the incidence of 10 strains of Streptococcus pneumoniae resistant to penicillin and other antibacterial substances in children under one year. In the majority otitis media was involved (8 x), in one instance with septic manifestations and detection of pneumococci also from the haemoculture. From a total number of 580 strains of Streptococcus pneumoniae isolated from the end of 1986 to the first half of 1988 the ratio of penicillin resistant strains was 1.72%. The values of penicillin resistance varied within the range of MIC from 1 mg/l to more than 16 mg/l. The strains were also resistant to ampicillin, oxacillin, cephalotine and tetracycline. One strain was moreover resistant also to erythromycin and lincomycin. There was also one strain resistant to cotrimoxazole with a MIC of 64 mg/ml.