Use of cytoplasmic 5'nucleotidase for differentiating malignant from benign monoclonal gammopathies. Bone marrow smears from 15 patients with multiple myeloma, 15 patients with monoclonal gammopathy of undetermined significance (MGUS), and 15 control subjects were examined for the presence of cytoplasmic 5'nucleotidase (c5NT) in plasma cells. Plasma cell positivity for c5NT (mean and 95% confidence interval) in patients with multiple myeloma numbered 46.4% (38.0-54.8%), in those with MGUS it was 15.3% (11.1-19.6%), and in control subjects it was 1.2% (0.3-2.1%). These findings indicate that c5NT can be used to differentiate benign from malignant monoclonal gammopathies. 