Administration of propylnitrosourea p.o. by our protocol induced a high incidence of hematolymphatic neoplasms in all six rat strains studied. Remarkable strain differences in susceptibility to thymic lymphomas were observed. The incidence of thymic lymphomas was high in Fischer 344 (98%) and Wistar/Furth (71%) but low in Sprague-Dawley (29%), ACI/Ms (23%), Donryu (24%), and Long-Evans (10%) strains. Segregation of thymic lymphoma incidence among crosses between highly susceptible Fischer and poorly susceptible Long-Evans rats indicated that the increased susceptibility to thymic lymphomas of Fischer rats was determined by a dominant gene TIs-1 (thymic lymphoma susceptible) and that this gene was linked to the coat color loci, p and c, in Linkage Group I in the order of TIs-1 - c - p. The presence of another independently assorting dominant gene, TIs-2, was also suggested to accelerate the thymic lymphoma-genesis. Expression of the group-specific antigen of murine leukemia virus as well as infectious viruses was not detected in nine propylnitrosourea-induced thymic lymphomas of Fischer rats.