Intracellular and secreted IgA from pokeweed mitogen (PWM)-stimulated normal peripheral blood lymphocytes, from 12-O-tetradecanoylphorbol-13-acetate (TPA)-stimulated peripheral blood lymphocytes of a patient with chronic lymphocytic leukemia (CLL), or from an IgA-producing human Epstein Barr virus (EBV)-transformed lymphoblastoid cell line were analyzed by molecular-sieve chromatography, electrophoresis in sodium dodecyl sulfate, and sucrose density ultracentrifugation. Fluorochrome-labeled anti-human IgA and secretory component (SC) were used as probes for the detection of polymeric IgA in individual cells. These methods demonstrated that the majority of intracellular IgA occurred in monomeric form, even when the predominant form of secreted IgA was polymeric. Sequential analyses of the IgA secreted by PWM-stimulated normal peripheral blood lymphocytes revealed that the proportion of the polymeric IgA increased with the time of culture and that polymers represented the prevalent form of secreted IgA from the fifth day of culture. Although approximately one-half of TPA-stimulated CLL cells bound fluorochrome-labeled SC, only trace amounts of extracellular and intracellular polymeric IgA were detected in both culture supernatants and lysates. Culture supernatants of an IgA-secreting EBV-transformed cell line contained predominantly polymeric IgA. However, intracellular IgA was largely represented by monomers. The predominance of intracellular monomers in polymeric IgA-secreting cells suggested that the pathway of the assembly of human IgA molecules is analogous to that described for mouse IgA synthesis.