Seventy percent of clinical isolates of penicillin-resistant pneumococci also exhibit defective lysis when treated with penicillin exceeding the minimal inhibitory concentration (MIC). To provide a possible explanation for the frequent association of these two traits, we exposed penicillin-susceptible pneumococci to two kinds of antibiotic pressures in the laboratory. Treatment of cultures with cycles of high concentrations of penicillin (20 X MIC) followed by growth of the survivors in drug-free medium selected for lysis-defective mutants that died only slowly during antibiotic treatment but had unchanged MICs. Exposure to sustained, low levels of penicillin produced resistant mutants, with elevated MICs, that lysed normally with penicillin. We suggest that the cyclic antibiotic exposure generally used in the clinical setting may select primarily for enhanced survival. From these survivors a second type of antibiotic exposure--sustained antibiotic concentrations just above the MIC (concentrations that may be restricted to the tail-end trough of a dosing interval)--selects for penicillin-resistant mutants.