Purified plasma cell suspensions were produced from 3 subjects with malignant plasma cell disease. Various physical, biological, and immunological properties of these cells were studied. Neoplastic human plasmablasts were found to be denser than their mature forms, in contrast to the usual relationship of the lymphoid, myeloid, and erythroid series. The human plasma cells did not respond to phytohemagglutinin stimulation and were found to be weakly reactive in mixed lymphocyte-plasma cell cultures. Potent antisera was produced against such cells, and the antisera demonstrated a broad cross reactivity with various human lymphocyte populations. The data suggest a lymphoid origin for human plasma cells and possibly a specific relationship to B-type lymphocytes.