The role of gastrin in the control of growth of renal G40 I cells isolated from a human nephroblastoma (Wilms' tumour) was investigated. G401 cell growth was enhanced in the presence of exogenous gastrin. Addition of anti-gastrin antibodies to serumfree medium significantly inhibited the growth of G401 cells. G40 I cells contained the equivalent of 4.3 pg/ I O6 cells of gastrin, and serum-free medium collected over 48 hr from G401 cells contained the equivalent of 38 ng/IO6 cells of gastrin, as determined by radioimmunoassay. Growth of G401 cells was inhibited in a concentration-related way by a variety of gastrin/ CCK receptor antagonists. Devazepide and proglumide were, respectively, the most and the least potent inhibitors of G401 cell growth (potency order devazepide > L-365,260 = lorglumide > loxiglumide > benzotript > proglumide). These gastrin/CCK receptor antagonists had similar growth-inhibitory activities in human colonic adenocarcinoma HCT-I I6 cells. Growth of HCT-I I6 cells was stimulated to a lesser extent, as compared with G401 cells, by exogenous gastrin, and endogenous gastrin was not detectable in HCT-I I6 cells. The results are consistent with a role for a gastrin-like peptide in the control of growth of a renal cell line. The data suggest that gastrin/CCK receptor antagonists warrant further investigation as therapeutic agents for the control of gastrin-responsive tumours derived from outside, as well as inside, the gastrointestinal tract, including tumours derived from the kidney.