A healthy 38-year-old white woman had two abortions and three live children. Red cells from each of her living children at birth had a strongly positive direct antiglobulin test. Detailed studies on the third child showed that the cells were sensitized by IgG. Maternal serum, tested by a range of techniques against reagent red cells and the husband's cells, showed no unusual antibody. Maternal serum and eluates prepared from red cells of the third child did not react with fresh cells from the older siblings (aged 7 and 10 years). Follow-up on the third child showed that the direct antiglobulin test was positive at 2 months, weakly positive at 4 months, and negative at 8 months. Red cells collected at 8 months of age did not react with the stored eluate prepared from the baby's sensitized red cells at birth. The most likely explanation of these data is that the children inherited a paternal antigen that is only present as a red cell surface-active structure during fetal development.