The sera of non-pregnant adult rabbits which had been hyperimmunized to Brucella abortus antigen, the sera of pregnant female rabbits which had not been immunized, and the sera, exocoelomic fluids, amniotic fluids and stomach contents of 25-day-old foetal rabbits were examined electrophoretically and ultracentrifugally. The serum of pregnant rabbits differed both in total protein concentration and in the proportions of the components from that of non-pregnant hyperimmunized rabbits. The foetal sera contained components corresponding to albumin, $\alpha -,\beta -$ and $\gamma -$ globulin, but the proportions of these components, as well as the total protein concentrations, differed widely from those of the sera of both pregnant and non-pregnant adults. Foetal exocoelomic fluid, amniotic fluid and stomach contents contain similar electrophoretic components and resemble each other closely in the proportions of the components, though differing in the total concentrations. The components resemble those of sera in mobility, but the proportions of the components differ widely from those of the sera, both foetal and adult. Various experimental procedures to which rabbits were subjected resulted, after 24 h, in a significant increase in the total protein concentrations, without any corresponding change in the proportions of the components, of the foetal sera. No corresponding changes were detected in the maternal sera. The effect on the sera could be explained by withdrawal of water alone from the foetal circulation. Using antibodies as markers, it was shown that both $\beta -$ and $\gamma -$ globulin enter the foetal circulation from immune rabbit serum injected into the uterine lumen. The importance of the contribution of maternal serum proteins to the foetus is discussed in the light of the immunological and electrophoretic results.