The presence of J chain in normal lymphoid cells (examined at various stages of differentiation) and in lymphocytes and plasma cells from patients with malignant hematologic disorders was determined by the immunofluorescence technique. J chain was not detectable in the cytoplasm or on the surface of either pre-B or B cells, but was present in a few peripheral blood lymphocytes that contained intracellular immunoglobulins of various classes. After pokeweed mitogen stimulation, intracellular synthesis of J chain and immunoglobulins appeared simultaneously and was most pronounced during early days of culture. These data indicate that J chain is not expressed during the earliest stages (pre-B and B cells) of differentiation of lymphoid cells and that the synthesis of J chain is closely linked to the production of immunoglobulin chains, irrespective of the class. Examination of lymphoid cells from bone marrow and peripheral blood of patients with various lymphoproliferative diseases (secretory and non-secretory multiple myeloma or Waldenstrom's macroglobulinemia, H- or L-chain diseases, chronic lymphocytic leukemia, plasma cell leukemia, and angioimmunoblastic lymphoma) revealed that J chain was present in various proportions of the cells that contained monomeric (IgG, IgD or IgA) or polymeric (IgA or IgM) immunoglobulins, as well as in cells from patients with H- or L-chain diseases. No correlation was apparent between the presence of J chain in lymphoid cells and their ability to secrete their product. Preliminary data indicate that J chain may participate in the intracellular assembly of immunoglobulins from their component chains.