A cultured renal carcinoma cell line, ACHN, continued to proliferate in the absence of exogenous growth factors supplied by fetal calf serum. ACHN cells had been previously used as the immunogen for the production of monoclonal antibody 5F4. Subsequent immunohistochemical studies showed that 5F4 strongly and selectively reacted with renal carcinoma cells compared with normal renal epithelium, suggesting that expression of the substance to which it was bound was amplified as a result of malignant transformation. Because renal carcinoma cells are relatively differentiated with regard to function except for elements to ensure sustained growth, the immunoreactivity patterns led to the hypothesis that 5F4 reacted with some component of a growth-stimulating pathway. The hypothesis was supported in the present work by showing that 5F4 specifically inhibited the growth of ACHN cells; ACHN cells prominently secreted a protein with a molecular mass of 178 kilodaltons that was immunoreactive with 5F4; the protein was partially purified by simple lyophilization of serum-free conditioned medium; both ACHN conditioned medium and the lyophilizate stimulated the growth of quiescent human fibroblasts and BALB/3T3 cells as well as serum-deprived human renal carcinoma cell lines ACHN, A498, Caki-1, and Caki-2. Specific immunoadsorption of the protein by affinity chromatography with 5F4 removed the growth-stimulating activity. The results demonstrated a novel growth-promoting substance secreted by ACHN cells with autologous activity as well as activity for human and murine fibroblastic cell lines and other renal carcinoma cell lines. The growth-promoting activities were specifically blocked by monoclonal antibody 5F4.