AbstractStable hybrids formed between clones of established murine T‐cell lymphoma lines, and between lymphoma clones and normal spleen or thymus cells were examined for their tumorigenic properties by intravenous (i.v.) and intradermal (i.d.) inoculation into syngeneic AKR mice. Fusion parents consisted of T lymphoma clones of high and low tumorigenicity derived from the SL 12 cell line. In addition, normal spleen cells and thymocytes were fused with poorly tumorigenic T‐lymphoma clones. Hybrids tested by i.v. inoculation of 106 cells to syngeneic hosts showed that fusion between the lymphoma cells resulted in hybrids which displayed the phenotype of the highly tumorigenic parent. Also, it was shown that fusion of poorly tumorigenic lymphoma cells with normal spleen cells resulted in hybrids with enhanced tumorigenicity. Fusion of poorly tumorigenic lymphoma cells with normal thymocytes resulted in hybrids with the highest tumorigenic potential. The pattern of spread for the tumor/tumor hybrid was that of the highly tumorigenic parent. Tumor spread patterns for the spleen/tumor hybrids were different from those of the thymocyte/tumor hybrids. Intradermal inoculation of 105 cells from tumor/spleen or tumor/thymocyte hybrids revealed differences in latent periods between parental and hybrid cells, the tumor/thymocyte hybrids having the shortest latent period. Surface marker studies and T‐cell antigen receptor mRNA determinations in the tumor cell/normal cell hybrids indicated that the normal parent was a cell of immature phenotype. Therefore, high tumorigenicity is a dominant characteristic, and poorly tumorigenic but “immortal” T lymphoma cells can derive characteristics which increase their In vivo growth capacity from the putative immature normal cells with which they selectively fuse.