Human tissues were examined for their ability to produce monomeric and polymeric forms of IgA in vitro. Mononuclear cells were examined for the presence of cytoplasmic immunoglobulins and J chain (by immunofluorescence) and for the amounts and molecular forms of intracellular and secreted IgA (by radioimmunoassay). Cultured bone marrow cells were the largest producers of total IgA, and approximately 90% of the secreted IgA was monomeric. Spleen cells also produced predominantly monomeric IgA. Intestinal lamina propria cells secreted the greatest amounts of polymeric IgA, but also produced significant amounts of monomeric IgA. Tonsillar, lymph node, and peripheral blood cells produced approximately equal proportions of monomeric and polymeric IgA. These results suggested that bone marrow may be a major source of serum monomeric IgA, and that lymphoid tissues associated with secretory surfaces may provide a greater proportion of polymeric IgA than other lymphoid tissues. There was a positive correlation between the production of polymeric IgA and the presence of cytoplasmic J chain; tissues that produced predominantly monomeric IgA displayed fewer cells containing cytoplasmic J chain.