Six-week-old female Sprague-Dawley rats were inoculated with a mammary tumor, MRMT-1. At 2 weeks after inoculation, one of the following 4 treatments was done: surgical tumor excision (SE), cryosurgery (CR), surgical excision plus re-inoculation of freezing-thawing-produced vaccine (FT), and surgical excision plus fasting for 72 hours (FA). In FT and FA groups, incidence of metastatic death were higher than in SE group, while that in CR group was similar as in SE group. Specific footpad reactivity at 2 and 3 weeks after treatment was lower in CR than in SE group. Winn's neutralization assay revealed that antitumor activity of spleen cells at 1 and 3 week(s) after treatment was lower in CR than in SE groups. In vivo observation on effect of inactivated serum at 1 week after treatment showed tumor enhancement in SE group and tumor inhibition in CR group. In conclusion, the observed mild reduction in antitumor immunity in the relatively early period after cryosurgery might not be due to blocking action of superfluous tumor antigens, but probably due to activation of suppressor cells consequent on cryosurgical stress or on slow absorption of tumor antigens.