Herpesvirus saimiri inoculated into owl monkeys (Aotus trivirgatus) causes leukaemia and lymphoma. Peripheral lymphocytes from leukaemic monkeys grown in culture are predominantly T lymphocytes. The supernatants from these cultures contain a factor which enhances the antibody response of murine B cells to sheep red blood cells (SRBC). The factor has been partially characterized by ammonium sulphate precipitation, DEAE-chromatography, gel filtration over Sephadex G-150 and disc-gel electrophoresis. The enhancing factor is a protein with a molecular weight of approximately 40,000.