Prostatic carcinoma is the most commonly diagnosed tumor among men over 40 which results in over 30,000 deaths each year in the United States. Previous studies indicated that tumor cell lines produce and release several growth regulatory factors into their condition media and so far a number of human tumor cell-derived suppressor factors have been isolated that affect normal immune functions. In this report, an immunosuppressive factor is identified from the supernatant of an androgen-dependent human prostatic carcinoma cell line (JCA-1). This factor is constitutively produced by JCA-1 cells and is able to suppress normal human peripheral blood lymphocyte proliferation irreversibly and in a dose-dependent manner. The immunosuppressive factor was semi-purified by a combination of ion-exchange chromatography on DEAESepharose and gel filtration (Sephacryl S-200) with an apparent molecular weight of 40-55 kDa. The immunosuppressive factor was not cytolytic to lymphocytes and was sensitive to 56°C, reducing agent as well as to proteinase digestion. Cell cycle analysis revealed that the immunosuppressive factor does not induce apoptosis, but is able to prevent G1 lymphocytes from entering into the S phase of the cell cycle. Further biochemical purification and immunological studies are needed to determine the importance of this factor and its relationship to the immune system.