After mutagenesis of mouse mastocytoma P815, it is possible to obtain variant (tum-) clones that are almost always rejected by syngeneic DBA/2 mice. Most tum- clones express new variant-specific antigens that can be detected by cytolytic T cells (CTL). Occasionally, mice injected with tum- variants eventually develop progressive tumors. Cells from these tumors were analyzed for antigen expression with variant-specific and P815-specific CTL clones. Out of 13 tumors examined, 11 were composed of cells that had clearly lost a variant-specific antigenic determinant. These results indicate that tum- variants induce a specific host rejection response which usually results in complete elimination of the variant cells, but that occasional antigen-loss constitutes an important mechanism of escape. The loss of a variant-specific antigenic determinant from one tum- clone that had escaped rejection allowed the detection of a residual variant-specific determinant. This was demonstrated by isolating a new CTL clone that lysed both the original tum- clone and its antigen-loss variant, but not other P815 targets. Thus, some complex transplantation antigens can be separated into independent determinants by using antigen-loss variants.