Three groups of adult male Nya:(FG) rabbits were treated intrarectally, 3 times a week for 7 months, with 1 ml of fresh homologous semen, 20 ml of colonic enema, or 20 ml of enema followed by 1 ml of previously frozen semen. A control group was untreated. The results of several in vitro tests of humoral and cellular response showed that chronic intrarectal insemination leads to an abrogation of the immune response, expressed differently in the two groups of rabbits receiving semen. A consistently depressed blastogenic stimulation in response to T-cell mitogens was observed only in the animals receiving fresh semen. Neither group produced antilymphocyte antibodies. Anti-seminal-fluid and antispermatozoa antibodies were detected after 3 months of treatment in two animals receiving enema and semen and one animal receiving fresh semen. Circulating immune complexes were detected after 2 months of treatment in animals receiving enema and semen. These immune complexes contained predominantly IgM and were specific for seminal antigens. Assay of the cellular response to a T-cell-dependent antigen showed, in the animals receiving enema and semen, a pronounced decrease in the indirect IgA and IgG plaque-forming cells response with practically no effect on the IgM response. These results, taken together with those of the preceding paper, suggest that in intrarectally inseminated rabbits the functional capabilities of the immune system may deteriorate without histologic or morphologic changes in the anorectal mucosa of relevant lymphoid tissues.