Chlamydia trachomatis, Mycoplasma hominis, and Ureaplasma urealyticum are genital agents that are being increasingly implicated in infectious pregnancy complications and abnormal pregnancy outcomes. We measured the in vitro activity of clindamycin against strains of these three agents which were isolated from pregnant women. For 30 strains of C. trachomatis, the median minimal inhibitory concentration was 1.0 microgram/ml (range, 0.25 to 2.0 micrograms/ml). For 27 strains of M. hominis, the median minimal inhibitory concentration was 0.12 microgram/ml (range, 0.06 to 0.25 microgram/ml) and the median minimal bactericidal concentration was 0.5 microgram/ml (range, 0.06 to 2.0 micrograms/ml). For 27 strains of U. urealyticum, the mean minimal inhibitory concentration was 4 micrograms/ml (range, 1.0 to 32.0 micrograms/ml) and the mean minimal bactericidal concentration was 32.0 micrograms/ml (range, 4.0 to 128 micrograms/ml). Thus in vitro clindamycin would appear to be highly active against pregnancy-associated strains of M. hominis, less active against strains of C. trachomatis, and least active against strains of U. urealyticum. Since M. hominis has been strongly linked to postabortal fever and to postpartum fever and endometritis, our results indicate that clindamycin should be evaluated in treatment trials in pregnancy aimed at prevention of M. hominis-induced morbidity as well as in treatment of the complications themselves.