One molecule of UTI68, a trypsin inhibitor purified from urine of healthy men, inhibited four molecules of bovine trypsin. This finding suggests the formation of various complexes of UTI68 with 1 to 4 molecules of trypsin. However, SDS polyacrylamide gel electrophoresis of the reaction products of UTI68 with trypsin showed that, at molecular ratios of UTI68 to trypsin of 1:1 to 1:3, UTI68 was rapidly cleaved by trypsin to form two proteins, Protein I and Protein II, whose molecular weights were estimated as 49,000 and 25,000, respectively, while at a ratio of 1:4, UTI68 was converted to Protein III with a molecular weight of about 30,000 and a smaller protein(s) than trypsin. These results were supported by gel filtration of the reaction products of UTI68 with trypsin on a Sephadex G-100 column at pH 3.0. Protein I and Protein II were separated, and Protein I was named UTI49. Protein II was separated from trypsin on a QAE-Sephadex column, and it had no inhibitory activity. Since one molecule of UTI49 inhibited about three molecules of trypsin, its interaction with trypsin was examined. On addition of one and two molecules of trypsin to one molecule of UTI49 at pH 8.0 complexes were formed consisting of one and two molecules of trypsin, respectively, with one molecule of UTI49, and both complexes were dissociated to their components at pH 3.0. Addition of three molecules of trypsin brought about further fragmentation of UTI49, and the split products formed a complex(es) with trypsin at pH 8.0, which dissociated at pH 3.0.