Benzanthracene-induced C57BU6 (H-zb) mouse T-cell lymphoma EL4 (a thymidine kinase-deficient cell line) was fused by using polyethylene glycol with an MIS* (MIS for minor lymphocyte stimulatory) antigen-dependent T cell line, which was designated G4 and had been derived from a C3HIHe mouse (H-2'9, and the fused cells were cultured in HAT medium. Although no growing cells appeared in most of these fusions, we consistently obtained growth-arrested H-2Kbpositive cells from the fused cell populations by the panning method. The cells were tetraploid and were able to proliferate in response to MIS' antigen. Three H-2Kb-positive clones, isolated by limiting dilution from three different fusions, were shown to be EL4 X G4 hybrids, because (I) they had both H-2* and H-Zb antigens; (2) each of the clones had one submetacentric chromosome which was a marker chromosome of EL4, and they were tetraploid with modal chromosome numbers of 74,78, and 79, respectively; (3) they had 4 isozymes of both parental cells. These results indicate that EL4 lymphoma cells cease to proliferate when fused with T cell line G4. The malignant phenotype of lymphoma EL4 is thus suppressed at the level of cell transformation by the introduction of the G4 cell genome.