The Cloudman (S91) melanoma elicits an immune response in syngeneic DBA/2 mice which prolongs the time before the tumor becomes detectable but does not significantly alter the growth rate subsequent to its appearance. The spleen can either enhance the immune response and prolong the lag phase when stimulated with high doses of tumor cells or suppress it and shorten the duration at lower doses. This bidirectional effect of the spleen is similar to previously reported activity of regulatory T cells which reside in the spleen. This observation is discussed in contrast to the role of the spleen in animals bearing tumors which elicit concomitant immunity.