Immune dysfunction in multiple myeloma. Reduced natural killer cell activity and increased levels of soluble interleukin-2 receptors. APMIS 99: 34G346, 1990.Multiple myeloma (MM) is characterized by an increased susceptibility to infections and to other malignancies. Selected related immune functions were studied. Spontaneous and interleukin-2-stimulated natural killer (NK) cell activities were normal in 19 patients with MM compared with 62 controls. In contrast, interferon-stimulated N K cells had a significantly lower increase in activity in M M than in controls. The normal improvement in lytic N K cell activity after addition of indomethacin to the mononuclear cell cultures (to inhibit prostaglandin-mediated suppression) was not observed in cultures from M M patients. As reported for other lymphoproliferdtive disorders, the levels of soluble interleukin-2 receptors in serum were significantly higher in M M (600 U/ml median value) compared with controls (317 U/ml median value), P < 0.0001, and the concentration of interleukin-2 receptors was significantly correlated with the concentration of monoclonal immunoglobulin in serum. Blood monocyte chemotactic responsiveness was significantly lower in M M patients with both zymosanactivated serum and f-Met-Leu-Phe as cytotaxins, suggesting reduced ability to accumulate at inflammatory foci. In contrast, release of reactive oxygen radicals, believed to be associated with the killing ability of monocytes, was normal after in v i~o stimulation.