The effects of pH on the formation and dissociation of the complex of trypsin and PSTI was investigated using pancreatic juice of two patients. Pancreatic juice was eluted from a Sephadex G100 column with the buffer in different pH values, and the trypsin inhibitory activity in the eluate was measured. Under alkaline and neutral conditions, the complex of PSTI and trypsin was observed, while in acidic condition, dissociation of the complex was occurred. Molar ratio of PSTI and trypsin in the complex was found to be one mole/one mole. Human serum, alpha 1-antitrypsin, and alpha 2-macroglobulin dissociated the complex of PSTI and trypsin.