Abstract Mixtures of human and dog pancreatic proteinases obtained from human pancreatic juice or extracts of canine pancreas were selectively inhibited using nine naturally occurring inhibitors of serine proteinases. Inhibition of trypsin, chymotrypsin, and elastase was monitored using specific synthetic substrates. Dog trypsin was inhibited by all nine inhibitors, while only four inhibitors significantly inhibited dog chymotrypsin and elastase. Inhibition spectra of human enzymes were completely different. Only three inhibitors exhibited significant inhibition of human trypsin. Human chymotrypsin and protease E (elastase) were inhibited only by 2 to 3 inhibitors and, in most cases, the amount of the inhibitor required to achieve full or 80% inhibition greatly exceeded the amount needed for the respective dog enzymes. Trasylol, the most widely used, commercially available inhibitor, inhibited both human and dog trypsins, but was totally ineffective against human and dog chymotrypsins and elastases. It was found that the most effective polyvalent inhibitors of human enzymes were chickpea and lima bean trypsin inhibitors. It was indicated by these results that strong species specificity exists and the invalidity of using animal models for the study of inhibition of human pancreatic proteinases was emphasized.