In 22 cases of myeloma and 12 cases of benign paraproteinemia the functional situation of circulating lymphocytes was studied. The technique used was the incorporation of 3H-Thymidine after PHA stimulation. E rosette formation and immunoglobulin bearing cells were evaluated. A reduction of 3H-Thymidine incorporation in the blastic cells was seen in the patients with myeloma (2,942 +/- 421 cpm/10(6) lymphocytes in respect to 13,182 +/- 920 of the control group). The difference was statistically significant: p less than 0.001. In benign monoclonal gammopathies the reduction was moderate and was at the limits of the significance: 9,482 +/- 1,100. The percentage of E rosettes and of immunoglobulins bearing cells was normal. The different behaviour of the blastic response in vitro in the two groups of patients tested confirms that the respective condition of the immunocompetent system is clearly different. A possible interpretation of this type of behaviour is discussed.