Four clones of pancreatic secretory trypsin inhibitor (PSTI)-overexpressing cells (TF-PANC clones 1, 6, 8, and 36) were established to evaluate the physiological function of PSTI secreted by cancer cells, by means of introducing a PSTI-expression vector (pRSV-PSTI) into the human pancreatic adenocarcinoma cell line (PANC-1). No obvious changes were observed in the histological features of these transplanted tumors in nude mice, in the growth of TF-PANC and PANC-1, or that of 3T3 fibroblasts when cocultured with them. Addition of recombinant human PSTI to the cultured media resulted in no increase in proliferation of fibroblasts (3T3 and WI-38) or of four pancreatic cancer cell lines (PANC-1, CAPAN-I, MIAPaCa-2, and Hs766T). These results suggest that the estimation of tumor-bearing PSTI as a paracrine or autocrine growth factor in recent studies should be given careful consideration.