Pancreatic secretory trypsin inhibitor stimulates the growth of rat pancreatic carcinoma cells. Pancreatic secretory trypsin inhibitor was examined for growth-promoting activity on five cell lines using standard cell culture techniques. One cell line, AR4-2J, derived from a rat pancreatic acinar cell carcinoma, responded with significantly increased incorporation of [3H]thymidine and colony formation. Pancreatic secretory trypsin inhibitor stimulated the incorporation of [3H]thymidine in liquid culture; the maximal increase was 61 +/- 10% above control (P less than 0.001) and was seen at a concentration of 10(-9) mol/L. Using a soft agarose clonogenic assay, pancreatic secretory trypsin inhibitor also consistently stimulated (3 assays) colony formation: the peak activity occurred at a concentration of 10(-10) mol/L which caused a 150 +/- 55% (mean +/- SE, P less than 0.05) increase above control. Aprotinin had no effect on the growth of AR4-2J cells and pancreatic secretory trypsin inhibitor did not bind to the epidermal growth factor receptor. AR4-2J cells were shown to produce pancreatic secretory trypsin inhibitor. The study raises the possibility that pancreatic secretory trypsin inhibitor provides autocrine stimulation of tumor cell growth. 