The activities of erythromycin and clindamycin against 350 Peptostreptococcus strains were studied during a 5-year period (1986 to 1991). In 5.1% of the Peptostreptococcus strains, which presented dissociated resistance (clindamycin MIC, less than or equal to 1 microgram/ml; erythromycin MIC, greater than 8 micrograms/ml), evidence of inducible macrolide-lincosamide resistance was shown. A total of 17.7% of the strains presented a constitutive phenotype; the clindamycin and erythromycin MICs for these strains were greater than 8 micrograms/ml.