Factors have been studied from human platelets that promote the growth of a hormone-responsive rat mammary adenocarcinoma cell line MTW9/PL, the BALB/c 3T3 mouse embryo fibroblasts, and numerous other established cell lines. A wide variety of the commonly employed cell lines, including lines of human, mouse, monkey, chicken, rat, Chinese hamster, and Syrian hamster origin, were tested for their growth response to a standard concentration of 200 microgram/ml human platelet lysate, and the lysate was found to contain mitogenic activity for 24 of the 29 different lines assayed. A comparison was made between the platelet growth activity for the MTW9/PL cells and the well characterized platelet mitogen for the BALB/c 3T3 cells, platelet-derived growth factor (PDGF). When the platelet lysate was subjected to digestion by highly purified trypsin, the mitogenic activity for the MTW9/PL cells was not affected whereas that for the BALB/c 3T3 cells was essentially destroyed. Crude PDGF was prepared by heating the human platelet lysates at 100 degrees C for 2 min followed by clarification, dialysis, lyophilization, and reconstitution. This PDGF material had no apparent growth activity for MTW9/PL cells, although chromatography of this material on Biogel P-100 revealed a high molecular weight (approximately 40,000 daltons) activity for the BALB/c 3T3 cells (presumably PDGF) and two growth activities for the MTW9/PL cells, one high molecular weight activity and a second activity of molecular weight less than 10,000. These studies demonstrated a form of epithelial tumor cell growth activity separable from the 3T3 type PDGF in crude heated extracts.