Stimulation of tumor cell growth by vasoactive intestinal peptide. Vasoactive intestinal peptide (VIP) stimulated the growth of murine Lewis lung carcinoma cells in culture. The growth promoting effect was dependent on the concentration of VIP. Exposure to VIP for 12 hours followed by removal of the peptide resulted in sustained growth promotion for 4 to 5 days in culture. Synthetic fragments of VIP, i.e., VIP (1-16) and VIP (22-28), and the unrelated peptide neurotensin failed to stimulate the growth of the Lewis lung carcinoma cells. The growth-promoting effect of VIP was also observed in a murine mammary tumor cell line and a human lung adenocarcinoma cell line. 