Four time-kill kinetic curves of S. pneumoniae strains incubated with erythromycin, penicillin, or cefotaxime, alone or in combination. (a) Serotype 12 (1014), fully susceptible. (b) Serotype 14 (039.0723), penicillin resistant. (c) Serotype 7 (315.0353), erythromycin resistant. (d) Serotype 6 (4828), both erythromycin and penicillin resistant. •, penicillin (four times the MIC); ▴, erythromycin (one-half times the MIC); , cefotaxime (four times the MIC); ○, erythromycin with penicillin; ◊, erythromycin with cefotaxime; ▪, control.