Previous studies have indicated that cells transformed by murine sarcoma virus (MSV) in the absence of virus production lack detectable transplantation antigens. Recent findings that antibodies to a surface antigen associated with MSV‐transformed cells can be elicited by hyperimmunization, led to the present studies. Attempts were made to detect transplantation immunity to MSV‐transformed cells in animals producing this antibody. Mice, hyperimmunized with an MSV‐transformed non‐producer clone of BALB/3T3, failed to demonstrate a higher resistance than BALB/3T3 hyperimmunized mice to tumor induction by either the homologous non‐producer cell or MSV itself. Under these conditions, both MSV non‐producer cell hyperimmunized tumor‐bearing and non‐tumor‐bearing mice showed antibodies to the MSV‐associated cell surface antigen. Thus, even in mice possessing this antibody, there is no transplantation immunity to MSV non‐producer cells.