OBJECTIVE: To elucidate the role of peritoneal macrophages in infertility, we investigated how the presence of peritoneal macrophages would affect fecundity in mice. Moreover, we also studied the effects of interleukin-1 on embryonic development. STUDY DESIGN: Mice were administered OK-432 intraperitoneally to induce macrophage infiltration of the peritoneal cavity; ovulation was then induced and animals were mated. On day 13 of gestation, fetuses were counted. After injection of OK-432 or interleukin-1, the mice were mated. Three days later, embryos were collected and the stage of embryo development was determined. RESULTS: In mice given OK-432 (n = 33), four (12%) became pregnant and the mean litter number was 6.0 +/- 3.6, whereas in the control group 23 of 30 mice (77%; p < 0.01) became pregnant and the litter number was 14.1 +/- 5.3 (p < 0.01). OK-432 and interleukin-1 administered intraperitoneally significantly suppressed embryo development (p