Immunoglobulin-secreting cells (ISC) in peripheral blood mononuclear cells (PBM) isolated from patients with plasma cell dyscrasia of various stages were studied using reverse hemolytic plaque assay. Normal healthy individuals contained 35 +/- 16 ISC/10(5) PBM. Aleukemic or subleukemic patients with overt myeloma contained 825 +/- 713 ISC, whereas leukemic patients contained 12,675 +/- 2520 ISC. Patients of premyelomatous stage, however, contained ISC/10(5) PBM of normal range. The relationship between ISC and clinical stage or laboratory data is discussed.