Mitogen-induced cellular cytotoxicity (MICC) of peripheral blood mononuclear cells (PBMCs) against K562 cell targets was assessed in 24 patients with multiple myeloma (MM) using the 24 hours 51Cr-release assay. We found that PBMCs from MM patients exhibited normal MICC values when cells were isolated, washed and cultured in vitro in the absence of patients' serum. Patients' serum inhibited MICC of normal PBMCs stimulated by PHA. A strong positive correlation was found between percentages of inhibition and the amount of serum paraprotein in the patients studied, suggesting that paraprotein should be the main inhibitory component in this model of cytotoxicity. The possible inhibitory effect of serum paraprotein of MM patients on other types of cellular immunity remains to be elucidated.