Plasmacytoma (PCT) cell lines dependent for proliferation and survival on a factor elaborated by the murine macrophage cell line, P388D1, were established in vitro. Adherent peritoneal cells induced by pristane produced 50-fold greater amounts of this activity in vitro than did resident cells. The molecules responsible for plasmacytoma growth were distinct from a number of characterized factors including interleukin-1, -2, and -3, macrophage colony-stimulating factor, B-cell stimulatory factor-1, B-cell growth factor II, epidermal growth factor, transforming growth factor-β, and γ- and β-interferon, none of which were able to support the growth of the factor-dependent PCT cell lines. These results suggest that PCT growth factor may be a novel factor that has not been previously characterized and, further, that its production is associated with the pristane-induced, chronic peritoneal inflammatory response that precedes plasmacytoma formation.