The microcytotoxicity assay has been used to study human maternal cell-mediated immunity against foetal lung cells derived from foetal autopsies and amnion cells from full-term deliveries. Cytotoxicity against semi-allogeneic foetal lung cells but not against adult skin fibroblasts was detected in fourteen cases. No cytotoxicity was detected in six mothers tested during the first 15 weeks of gestation whereas 58% of mothers in the 15-17 weeks of gestation (twelve cases) and 88% with more advanced pregnancies (eight cases) were specifically cytotoxic. Cross-reactions against allogeneic foetal lung cells were mainly produced by effector cells from patients with 16-17-week-old pregnancies. No cytotoxicity was detected against semi-allogeneic amnion cells by effector cells from mothers after full-term deliveries. Cytotoxicity was blocked by autologous post-abortum sera in two out of seven cases. The effect of autologous post-abortum sera on non-cytotoxic effector cells was nonspecifically arming in four out of ten cases.