Normal rat kidney cells infected by a variety of transformation-defective temperature-sensitive avian leukosis sarcoma virus mutants were tested for the expression of transformation characteristics at permissive and restrictive temperature. Morphology, growth behaviour and agglutinability by concanavalin A corresponded fully to the phenotype of the infected cells: at permissive temperature the cells resembled wild type virus transformed cells, whereas when grown under restrictive conditions they became virtually indistinguishable from normal cells. The quantitative expression of allo- or xenogeneic cell surface antigens was not significantly affected by the phenotype of the cells. Two out of the five tested mutants induced tumour antigens in the expected temperature-dependent manner, whereas the other three mutants were able to induce tumour-specific cell surface antigens even in the revertant cells cultured at the restrictive temperature. These findings extend previous results about tumour antigen induction in mutant-infected cells of the natural host, the chicken embryo fibroblasts. The value of transformation-defective tumour antigen-positive mutants for vaccination purposes will be discussed.