Polylysines of 7–371 lysine residues inhibited pepsin over the pH range 3·6‐5·0 in a system using azocoll as substrate. Tetralysine was inactive. An almost 1: 1 molar ratio with pepsin gave maximum inhibition for a polylysine containing 59 lysine residues but increase in polylysine molecular size beyond this size was not accompanied by increase in activity on a weight basis although the polylysine: pepsin molar ratio for maximum inhibition decreased and inhibition mechanism varied. Polylysines of 59 and 158 lysine residues which were intermediate in the series were non‐competitive inhibitors, whilst polylysines of greater and smaller molecular size were competitive inhibitors, although only the smallest inhibitory polylysine, containing 7 lysine residues, was a pure competitive inhibitor. Polylysine inhibition of pepsin was found to be strongest at pH 5·0 and the pH dependence appeared to be associated with the relative concentrations of the enzyme and inhibitors in ionized form. For each polylysine it was possible to detect a polylysine: pepsin concentration ratio for which inhibition was pH‐independent over the range 3·6‐5·0.

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