Using a commercially available, competitive ELISA kit based on a polyclonal anti-interleukin-10 antibody, serum interleukin-10 (IL-10) levels were quantified in samples of different groups of patients: 20 healthy controls (CTR), 11 monoclonal gammopathies of uncertain significance (MGUS), 17 multiple myelomas (MM), 10 cancer patients (CANCER), 13 cancer patients + MGUS (MGUS-CA) and 7 MGUS patients after surgical removal of concomitant cancer (MGUS-SRCC). Results show significant differences of both the median levels of IL-10 and the monoclonal component (MC) in CTR, MGUS and MM (patients with increasing concentrations in the mentioned order). The IL-10 levels found in the three groups of cancer patients showed serum levels higher than those observed in the controls. Moreover, the surgical cancer removal was related to an IL-10 decrease. A higly significant correlation between serum IL-10 levels and the corresponding MC was also found in the MM-bearing patients and to a lesser extent, in MGUS patients, indicating that serum IL-10 is parallel to the amount of the activated clone causing the monoclonal gammopathy. Since human myeloma lines, cultured in vitro may release significant amounts of IL-10, the data presented support the hypothesis that serum IL-10, measured in myelomatous patients may, at least in part, derive from the activated clone causing the monoclonal gammopathy.