Morphological alterations were observed after 5-15 serial passages of cells infected with three different strains of avian leukosis virus: OK 10, an A subgroup virus isolated from a natural infection; RAV-1, an established laboratory strain of the A subgroup, and RAV-2, a laboratory strain of the B subgroup. The infected cells had a prolonged lifespan of approximately 28 passages, compared to 14 passages for control cells. However, the altered cells had none of the attributes of transformed cells, such as growth in soft agar, loss of contact inhibition, tumor formation in chickens, or loss of fibroblast surface antigen. Therefore, we refer to the changes as conversion rather than transformation. The morphological changes differed depending upon the subgroup of the inducing virus. Cells converted with the A subgroup viruses were uniformly epitheloid whereas cells converted with the B subgroup virus were less uniform in size and shape. We speculate that an event similar to conversion may take place in vivo and contribute to the oncogenicity of leukosis viruses.