We have obtained cytolytic T-T hybrids by fusing an H-2Kk restricted clone specific for the hapten 3-(p-sulfophenyldiazo)-4-hydroxyphenyl acetic acid (SP) with an H-2Dd-restricted clone specific for the hapten fluorescein (FL). Several hybrid clones express both parental specificities but fail to lyse SP-coupled H-2Dd and FL-coupled H-2Kk target cells. We also fused the H-2Kk-restricted, SP-specific clone with a clone which recognizes FL in the context of any class I major histocompatibility complex antigen. Again several hybrids show both parental specificities but fail to recognize SP coupled to target cells which are not recognized by the parental SP-specific clone. These findings indicate that the observed cytotoxic T lymphocyte specificities for haptens on the one hand and polymorphic as well as nonpolymorphic class I major histocompatibility complex antigenic determinants on the other hand are not carried by independent proteins.