The cytoplasmic affinity of polymeric IgA for secretory component (SC) and the expression of joining (J) chain were examined in pokeweed mitogen (PWM)stimulated human peripheral blood lymphocytes (PBL) to determine, on the ultrastructural level, the polymerization sites of human IgA. SC-binding was found in 5.7% of transformed PBL on day 7 of culture; SC-binding was observed in a high proportion of IgA-produc-ing cells. A low proportion of IgMproducing cells also bound to SC, while there was virtually no SC-binding by IgGproducing cells. A high proportion of IgA- and IgMproducing cells expressed intracellular J chain, while approximately half of the IgGproducing cells were positive for J chain. The number of J chain-positive cells exceeded the number of SC-binding cells among transformed PBL on day 7 of culture. Immunoelectron microscopic study of the sites of SC-binding, and of IgA and J chain expression, revealed that polymerization of human IgA and the addition of J chain occur in the perinuclear space and endoplasmic reticulum, prior to immunoglobulin secretion. © 1990, Japan Society for Cell Biology. All rights reserved.