Protease inhibitors have been shown to be effective suppressors of carcinogenesis in vitro and in vivo. For example, the potato-derived chymotrypsin inhibitor 1 (CI-1) suppresses radiation transformation of C3H/10T1/2 cells in vitro. In the current study, we have investigated the interaction of CI-1 with C3H/10T1/2 cells. At the concentrations examined, CI-1 was non-toxic and had no effect on the doubling time or saturation density of these cells. This compound was taken up by these cells in a time dependent manner. Analysis of CI-1 from treated cells on a chymotrypsin affinity column revealed that active inhibitor was present in the cells. Additionally, CI-1, as well as the soybean derived Bowman-Birk inhibitor and chymostatin, blocked the cleavage of the peptide substrate Suc-Ala-Ala-Pro-Phe-MCA by intact C3H/10T1/2 cells. We have previously demonstrated that this substrate will reduce the transformation yield following treatment of cells with ionizing radiation. Our results suggest that CI-1 may inhibit transformation of C3H/10T1/2 cells in vitro by inhibiting the activity of Suc-Ala-Ala-Pro-Phe-MCA hydrolyzing activity in these cells.