The effects of syngeneic (CBAxCBA) and allogeneic (CBAxC57/B1) pregnancy on immunoglobulin (Ig)-secreting cells in various maternal organs/tissues have been investigated by using the protein A plaque assay. The following organs/tissues were examined: a) spleen, b) cervical nodes, c) inguinal nodes, d) mesenteric nodes, e) para-aortic (uterus-draining) nodes, f) Peyer's patches, and g) bone marrow (femur). The changes observed were similar and of the same magnitude in all pregnant animals, irrespective of the type of mating. At mid-gestation (day 14) a distinct increase in Ig secretors was observed, predominantly in the spleen. At the end of pregnancy (day 20) the para-aortic nodes contained dramatically increased numbers of plaque forming cells. A slight increase in both IgM and IgG-secreting cells was also seen in bone marrow at the very end of pregnancy, while Peyer's patches and the nodes of neck and legs appeared to be unaffected throughout the period of gestation.