To study antifertility properties of microbial toxins, exoenterotoxin and endotoxin from Vibrio cholerae were injected intravenously into mice at different times during pregnancy. The two substances induced termination of pregnancy, but the patterns of abortifacient activity were different. Exotoxin terminated pregnancy in mice when administered between Days 4 and 10 of gestation, but abortifacient activity was reduced in animals more than 10 days pregnant; exogenous progesterone did not protect the pregnancies. Endotoxin was most effective in terminating pregnancy when injected after mid-gestation and the active principle was heat-stable; exogenous progesterone was not able to prevent the effects of endotoxin. Animals treated with endotoxin on Day 17 often gave birth to live young prematurely; indomethacin reduced the incidence of premature littering. The results demonstrate that exo- and endotoxins have antifertility properties and both appear to act on intrauterine targets rather than inducing progestin deficiency.