Dimeric human secretory IgA was completely reduced with mercaptoethanol and alkylated with [14C] iodoacetamide. The component polypeptide chains were separated by high performance gel filtration in 5 M guanidine HCl into two fractions: one containing secretory component (SC) + heavy (H) chains; and the second containing light (L) + J chains. L and J chains were subsequently separated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS) or in alkaline urea. Calculations of the J chain stoichiometry in the dimeric secretory IgA (S-IgA) molecule were based on: (1) the measurement of the ratio of radioactivities of SC + H chain and L + J chain-fractions or L chain-and J chain-fractions; (2) the known stoichiometry of SC, H and L chains; and (3) the known number of half-cystine residues in the component polypeptide chains of S-IgA molecule. The data demonstrated that one molecule of dimeric S-IgA contains approx. one J chain. © 1986.