The involvement of the immune system in changes in luteal function was evaluated in rabbits. Pseudopregnancy was induced in 10 females and the spleens (considered to be the source of macrophages) of five were excised on day 7 of pseudopregnancy, while the five controls had sham splenectomies. Subsequent changes in serum progesterone concentrations were measured as an indicator of luteal function and luteolysis. A second pseudopregnancy was induced 31 days after splenectomy. The first pseudopregnancy was prolonged and during the second pseudopregnancy the serum progesterone concentrations on days 3 and 7 were much lower in the splenectomy group than in the control group. On day 14 of the second pseudopregnancy, the serum progesterone concentration markedly decreased in the control group while it remained almost at the level of day 7 in the splenectomy group. These results suggest that splenectomy suppresses the expression of luteal function and delays luteolysis in rabbits.