Soluble suppressor factor from the spleens of tumor-bearing mice

Spleen cells from tumor-bearing mice when cultured for 3 to 5 days released a soluble factor into the media that suppressed the stimulation of lymph node and spleen cells by tumor antigen or mitogens. Spleens from mice bearing MC43 tumors for 14 days were capable of producing suppressor factor in vitro, while those from mice bearing the tumor for 10 days or less failed to do so. Lymph node cells from the same animals did not produce suppressor factor in vitro. The suppressor factor was produced by a nonadherent cell population, was heat stable, was lost on dialysis, and did not appear to be tumor antigen or thymidine.