Serum from 70 patients with sporadic inclusion body myositis (IBM) was subjected to agarose gel immunofixation electrophoresis. The IgG extracted from 9 patients with monoclonal proteins, 3 without, and 2 control subjects and was purified, biotinylated, and applied to muscle biopsy sections for immunocytochemistry and to purified muscle protein fractions for immunoblots. Sixteen of 70 (22.8%) patients with IBM, compared with 2% of age-matched controls, had a monoclonal gammopathy characterized as IgG lambda in 9 patients, IgG kappa in 4, IgM kappa in 2, and IgA lambda in 1. The mean age of IBM patients with gammopathy was 60.6 years (range, 35-77 years), compared with 66.1 years (range, 42-80 years) of the IBM patients without gammopathy. The IgG of the patients, more often than that of the control subjects, immunostained myonuclei and recognized various muscle proteins of 35 to 145 kd. We conclude that IBM, regardless of age, is frequently associated with monoclonal gammopathies, which often recognize various muscle components, especially myonuclei, suggesting disturbed immunoregulation.