Résumé : After mutagenesis of mouse mastocytoma P815, it is possible to obtain at high frequency stable tumor cell variants (tum-) that are rejected by syngeneic DBA/2 mice. Most of the variants express one or more new individual antigens specific for each variant, that are detectable in vitro by cytolytic T cells (CTL). Somatic hybrids were prepared either between tum- variants and the original P815 clone, or between different variants. Antigen expression of the hybrids was assessed by using long-term CTL clones that recognize specifically the new antigen present on the variants. Expression of tum- variant antigens behaved as a dominant trait in the hybrids. By submitting the somatic hybrids to selection with CTL clones, it was possible to obtain antigen-loss hybrid variants. The analyses of these antigen-loss variants showed that two variant-specific antigenic determinants associated with one of the variant fusion partners could be lost independently. Like the parental tum- variants, both the (tum+ X tum-) and (tum- X tum-) hybrids failed to form tumors in normal mice but formed tumors in irradiated mice.