We introduced an IL-6 cDNA expression vector into a murine B cell line, the growth of which definitely required the presence of exogenous IL-6. The transfected cells secreted substantial amounts of IL-6, to which they themselves responded by proliferating without further requirement of exogenous IL-6. The proliferation was a direct function of cell density and was inhibitable by antibodies to IL-6, indicating the autocrine nature of the growth. The IL-6 cDNA-transfected cells displayed greatly enhanced tumorigenicity when inoculated into syngeneic and nude mice. Our data suggest that an IL-6 autocrine self stimulation confers on B cells a selective growth advantage and results in the induction of progression of the malignant state of B cells.