Serum β2-microglobulin concentrations were determined in samples of 65 patients with benign or malignant monoclonal gammopathy. In the group of patients suffering from multiple myeloma or Waldenström’s macroglobulinemia the mean β2-microglobulin level was significantly higher than in the group with benign monoclonal gammopathy. Values above 3 mg/l were highly indicative of malignant disease and observed in 50% of the myeloma patients. Serum creatinine levels were significantly correlated to β2-microglobulin levels. However, mean creatinine concentrations did not significantly differ between the two groups of patients. Plasma cells and lymphoplasmocellular elements containing cytoplasmic immunoglobulin were counted in bone marrow samples of all patients. The counts, expressed in percent of nucleated bone marrow cells, allowed a good discrimination between the benign and the malignant group of patients. Bone marrow from patients with multiple myeloma or macroglobulinemia contained more, from patients with benign monoclonal gammopathy less than 17% plasma cells. No significant correlation was noticed between the extent of this plasmocytic bone marrow infiltration and serum β2-microglobulin or creatinine levels.