Two polyene antibiotics, nystatin and amphotericin B, were found to be mitogenic for mouse spleen cells as measured by induction of DNA synthesis and polyclonal antibody production. This effect was demonstrated on spleen cells from nude‐mice and anti‐θ‐treated spleen cells tram normal mice. No effect was found on cortisone‐resistant thymocytes or on spleen cells treated with anti‐mouse bone marrow‐derived lymphocyte antigen antiserum. Nor was there any effect on spleen cells passed through a nylon fiber column. Thus we conclude that nystatin and amphotericin B are murine B‐cell mitogens.