Syngeneically and allogeneically mated CBA mice were daily given orally either pure peanut oil or peanut oil containing 0.5 mg 2,2', 4,4', 5,5'-hexachlorobiphenyl (HCB) beginning at the day of implantation. The mice were dissected on day 12 and 18 of gestation when the weight of thymus, spleen, and liver were recorded. The spontaneous mitotic activity of spleen cells was evaluated in vitro, and the number of resorbed foetuses was recorded. Neither spleen weight nor thymus weight was altered, but the liver weight was significantly increased by HCB treatment. The spontaneous mitotic activity of spleen cells did not differ significantly between HCB-treated and control mice. The frequency of resorbed foetuses was not increased by HCB treatment either in syngeneically or allogeneically mated mice, but it was observed that mice fed HCB were less sensitive to environmental disturbance than control mice.