In 28 cases of amyloidosis, bone marrow was examined to determine whether detection of clonal excess of plasma cells could be used to distinguish immunoglobulin-derived amyloidosis from other forms of amyloidosis. Patients were selected from a group of 315 cases of biopsy-proved amyloidosis in which bone marrow aspiration and biopsy had been done. There was no detectable monoclonal immunoglobulin in the serum or in the urine in 28 patients. Of 13 patients with immunoglobulin-derived amyloidosis, a clonal excess of plasma cells in the bone marrow could be detected in 11 (sensitivity, 85%), even though no immunoglobulin could be found circulating in the serum or urine after immunofixation with two light chain antisera of different sources. The 15 remaining patients had other forms of amyloidosis; 14 had polyclonal populations of plasma cells in the bone marrow (specificity, 93%). The positive predictive value of clonal excess was 92% and its negative predictive value was 88%. We conclude that, in the subset of patients with amyloidosis who do not have a circulating M component in the serum or urine detection of a clonal excess of plasma cells is useful in characterizing the type of amyloidosis. Correct classification is important for appropriate assessment of prognosis and therapy for the group of patients.