The catalytic action of papain (EC 22.214.171.124) produces insoluble polyleucine peptides, eight to nine units long, from leucine methyl ester. This reaction is preceded by a pronounced induction period. It can be concluded from the dependence of the reaction rate and of the length of the induction period upon monomer concentration and pH, that unprotonated leucine methyl ester is the reacting species. The induction period is shortened, though not abolished, by the addition of dimer. Addition of a low concentration (0.02 mM) of trimer abolishes the induction period. At higher concentrations of added trimer a higher initial rate is observed, owing to fast growth of trimer to octamer. The phenomena can be explained by the combinaton of: 1. (i) a very slow dimerization rate and a high rate of chain growth from the trimer onwards; 2. (ii) the occurrence of "feedback" reactions, which produce new peptide esters of sufficient length to grow fast; 3. (iii) precipitation of the final product. A highly simplied mathematical model confirms the explanation.