We investigated the role of Interleukin-6 (IL-6) as an autocrine growth factor for the retroperitoneal fibromatosis (RF) cells present in macaques infected with the simian retrovirus type 2 (SRV-2). Elevated levels of IL-6 were found in serum of SRV-2 antibody-positive macaques, ascites from RF-positive animals, and RF cell line culture media. IL-6 mRNA levels increased approximately five-fold in RF cells incubated with exogenous SRV-2. In RF cells, SRV-2 functions to increase IL-6 mRNA and protein production and presumably serves as autocrine growth factor.