Histamine has recently been shown to be a growth factor for some gastric and colorectal cancer cells. Previous studies have shown that cimetidine blocks in vitro and in vivo histamine-stimulated growth and cAMP release from the human colonic cancer cell line, C170. In this study, ranitidine, another H2 receptor antagonist, did not affect either basal or histamine-stimulated in vitro proliferation of C170, and failed to prevent cAMP release in vitro. Ranitidine did not inhibit in vivo growth of C170 at a dose of 1, 10, 25, 50 or 100 mg/kg, in contrast to 50 mg/kg/day cimetidine, which produced 39.3% inhibition of tumour volume (p<0.01) after 23 days' treatment. Ranitidine did not inhibit in vivo histamine-stimulated growth of C170 cells . LIM2412, another colonic cancer cell line, was significantly stimulated by both cimetidine and ranitidine in vivo. Ranitidine had no effect on in vitro cell proliferation.