Noninbred Sprague-Dawley rat embryo cell clones predictably undergo transformation after 20-30 in vitro passages following spontaneous release of endogenous rat leukemia virus (RaLV). In the presence of RaLV-specific antiserum, virus production and infectivity were reduced and transformation was delayed from 6 to 25 weeks. Transformation was not associated with an increased expression of Kirsten murine sarcoma virus-related src gene RNA.