A retrospective study was carried out to determine the diagnostic and prognostic value of the soluble form of the embryonal neural cell adhesion molecule NCAM (CD56) in paraproteinemia. NCAM, beta 2-microglobulin, and interleukin-6 levels were measured in the sera of 170 patients with paraproteinemia. Of these, 125 had multiple myeloma, 20 Waldenström's disease, and 25 monoclonal gammopathy of unknown significance. Serum NCAM proved superior to beta 2-microglobulin and interleukin-6 in distinguishing multiple myeloma from paraproteinemias of various causes, with a high specificity of 95.5% in detecting multiple myeloma, although with a low sensitivity of 40%. In multiple myeloma NCAM is significantly correlated with beta 2-microglobulin, paraproteinemia, and low albumin levels. For the determination of tumor load beta 2-microglobulin levels accord best with the Salmon and Durie classification scheme. When patients are grouped according to their clinical course, the disease development is reflected better by NCAM than by beta 2-microglobulin or interleukin-6. Survival data indicate that all three markers have prognostic potential. Prognostic accuracy with respect to overall survival is best with beta 2-microglobulin.