Previous studies have demonstrated that J chain is expressed not only in cells that produce polymeric immunoglobulins, but also in those engaged in synthesis of monomers including IgG and IgD. The presence of J chain in these cells suggested that its role may not be restricted to the formation of polymers. For the present study, fluorochrome-labeled polyclonal anti-J-chain and monoclonal antibodies to IgG subclasses were used to determine the distribution of J chain in IgG plasma cells from normal human tissues and from pokeweed mitogen (PWM)-stimulated human peripheral blood lymphocytes. The results indicate that J chain is not equally distributed among cells producing different IgG subclasses. The percentages of PWM-stimulated cells containing J chain were: 22 ± 5 (SE) for IgG1, 49 ± 6 for IgG2, 17 ± 7 for IgG3, and 64 ± 11 for IgG4. Examination of sections of various human lymphoid tissues revealed that the frequency of IgG cells that coexpressed J chain was lower than that observed in the PWM system and displayed variable distribution among IgG subclasses. The frequency of J-chain expression in IgG-producing cells may be related to the degree of cellular maturation and may differ according to the origin of cells. © 1985.