For unknown reasons, non-specific stimulation of the maternal immune system in pregnant mice has what appears to be a broad-spectrum efficacy for reducing birth defects. Immune stimulation by diverse procedures has proven effective, including footpad injection with Freund's complete adjuvant (FCA), intraperitoneal (IP) injection with inert particles to activate resident macrophages, IP injection with attenuated Bacillus Calmette-Guerin (BCG), and intrauterine injection with allogeneic or zenogeneic lymphocytes. Morphologic lesions that were significantly reduced included cleft palate and associated craniofacial defects, digit and limb defects, tail malformations, and neural tube defect (NTD). Teratogenic stimuli to induce these lesions included chemical agents (2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD], ethyl carbamate [urethane], methylnitrosourea [MNU], cyclophosphamide [CP], and valproic acid [VA]), physical agents (X-rays, hyperthermia), and streptozocin (STZ)-induced diabetes mellitus. Limited information is available regarding mechanisms by which such immune stimulation reduced fetal dysmorphogenesis. The collective literature suggests the possibility that immunoregulatory cytokines of maternal origin may be the effector molecules in this phenomenon.