Successful reproduction involves exposure of the female in significant antigenic challenge. These experiments were done to test the possibility/hypothesis that seminal fluid has an immunosuppressive role in the early stages cf pregnancy, which cculd pretect the conceptus from immonological attack. A highly inbred strain cf rat AO(RTI") was used in this study. Eleven groups of animals were examined, one during the estrous phase of the ovarian cycle to provide base-line data, five during pseudopregnancy, designated as controls, and five groups during syngeneic pregnancy. Each group contained a minimum of six animals. For each animal, body weight and organ/tissue weightsocf the uterus, thymus, uterine and popliteal lymph nodes, were recorded together with total cell counts of the lympocid tissues and their proliferating cell counts. The material was collected during each of the first five days of pregnancy and pseudopregnancy. The present study supports an immunological suppression by seminal fluid during early pregnancy as shown by a significant sustained depresson of the proliferative lymphocyte response in the uterine regional lymph nodes.