In the present experiments the changes that occur in the central and peripheral lymphoid organs during a first pregnancy in the mouse have been followed up in BALB/c females (H-2d) mated either to males of the same inbred strain or to C3H/Bi males, a strain differing at the major histocompatibility locus (H-2k). Three parameters were examined: weight, morphology and DNA synthesis. The results to be presented strengthen existing evidence that sensitization of the female to foetal transplantation antigens of paternal origin may occur during a first pregnancy and suggest that this process develops at least in part via the lymph nodes draining the uterus.