We analysed the reactivities of IgM and of IgG towards protein antigens in normal human tissue extracts in the sera of healthy women in the third trimester of pregnancy, young nulliparous women and adult men. The self-reactive antibody repertoires of IgM in the serum of pregnant women exhibited striking homogeneity among individuals and did not differ from those of nulliparous women and young males. Similar results were obtained upon analysis of repertoires of IgG purified from serum. Multiparametric statistics failed to discriminate among pregnant women, nulliparous women and young adult males for reactivity of serum IgG and purified IgG with self-antigens. The results indicate that self-reactive IgM and IgG repertoires do not differ in the third trimester of a normal pregnancy from those which characterize healthy adults.