Mice infected with lactate dehydrogenase-elevating virus (LDV) developed antibodies reactive with a tumour cell surface antigen (TSA) of Moloney sarcoma virus (Mo-MSV)-transformed mouse cells (Sac). We demonstrate that STU mice infected with LDV were protected against growth of syngeneic Sac tumour cells as early as 23 days post-infection (p.i.) and up to 5 months p.i. Nine LDV strains, including the neurovirulent LDV-C, elicited production of anti-TSA antibodies, which were restricted to the IgM isotype. Monoclonal anti-TSA antibodies were raised 4 days after infection of STU mice with LDV. When tested against several transformants of STU and BALB/c mouse origin they were found to react with Mo-MSV transformants (PV-TC-77, STU mouse origin; MSV85 C1 3, BALB/c mouse origin), methylcholanthrene-transformed MethA cells (BALB/c origin) and L929 cells. We suggest that the well known tumour growth inhibition by LDV is due to LDV-induced anti-TSA antibodies.