Serum from hamsters immunized repeatedly with cytomegalovirus (CMV) was able to block spleen-cell cytotoxicity for CMV-infected and CMV-transformed cells. Inhibition was observed regardless of whether spleen cells were obtained from hamsters sensitized to the virus or to isografts of the transformed cells (designated Cx-90-3B, T-2). Such serum did not significantly block effector cell response against transformed cells expressing herpes simplex virus or SV40-associated membrane antigens. The mechanism of blocking was investigated. It was found that the blocking activity could be adsorbed out with Cx-90-3B, T-2 cells but not by untransformed hamster embryo fibroblasts. Cx-90-3B, T-2 target cells treated with serum, and then washed, remained resistant to effector cell attack. On the other hand, if serum-treated attacker cells were washed their cytotoxic activity was not significantly impaired. These results suggest that the blocking factor in the serum is antibody-directed against cytomegalovirus-related membrane antigen. This conclusion is supported by the finding that the serum contained antibody specifically reactive with the transformed cell surface in isotopic antiglobulin tests.