Antibodies against CD8 or CD4 antigens can prevent T-cell functions induced by T-cell targets. As CD8 or CD4 antibodies can also initiate negative signals in T cells in the absence of appropriate targets it is not clear whether CD8 and CD4 molecules are directly involved in the interaction of T cells with their targets. In previous experiments we have introduced the T-cell receptor alpha- and beta-chain genes from a CD8-positive cytolytic T cell specific for the antigen fluorescein (FL) and the H-2D molecule of the major histocompatibility complex (MHC) into a CD8-negative recipient cell. The CD8-positive donor cell lysed both FL-conjugated fibroblasts and lymphoblasts, which express relatively high and low amounts of H-2D molecules, respectively. In contrast the CD8-negative transfectant lysed FL-conjugated fibroblasts only. Here we show that recognition of FL-conjugated lymphoblasts by the transfectant is enhanced by supertransfecting it with the CD8 gene.