Our earlier observations indicate the possibility of involvement of a ‘host factor’ in pregnancy-induced protection against mammary carcinogenesis. The present investigation is an attempt to determine if this ‘host factor’ is of immunological nature. Rat mammary tumors induced by 7,12-dimethylbenz[a]anthracene were used as target, and splenocytes from parous rats of the same strain were used as effector cells. The parous rats were divided into two groups according to the time period after parturition. Group 1 contained rats which were 5–13 days after delivery, group 2 had rats 14 or more days after parturition. In vitro cytotoxicity was determined by incubating the tumor cells with spleen cells in the target:effector ratios of 1:3, 1:10 and 1:30. Both groups 1 and 2 showed significant lysis of mammary tumor cells. These results were confirmed by the trypan blue exclusion test. The results showed a significantly higher cytotoxic capability of the spleen cells from parous rats against mammary tumor cells as compared to spleen cells from age-matched nullipa