The study of monoclonal gammopathies in the elderly provides an opportunity to define immunologic and neoplastic changes with aging. Previous reports using paper and cellulose acetate electrophoresis have documented an age-related increase in monoclonal gammopathies. In this study, the more sensitive techniques of high-resolution agarose gel electrophoresis and immunofixation were used, in conjunction with other protein studies, to further evaluate the prevalence of monoclonal gammopathies in 111 ambulatory residents (aged 62 to 95) of a retirement home. Eleven of the 111 residents (10 percent) were found to have a monoclonal gammopathy, ranging in concentration from 0.2 to 1.8 g/dl. All monoclonal gammopathies were confirmed by immunofixation, which also documented the presence of additional unsuspected monoclonal components in three of the 11 residents. The prevalence of monoclonal gammopathies by age ranged from 6 percent in the group younger than 80 years of age to 14 percent in the group older than 90 years of age. Only one of the 11 residents had any clinical or routine laboratory suggestion of a monoclonal gammopathy. The other 10 had normal ratios of total protein and albumin to globulin. Five of the 11 (45 percent) had an otherwise clinically unexplained erythrocyte sedimentation rate of more than 20 mm/hour, compared with only two of 100 in the group without monoclonal gammopathies. Follow-up studies one to three years after initial evaluation revealed that five of the 11 patients had died, two with evidence of disease progression. In the other six patients, monoclonal protein concentration and other protein values remained stable. An unexplained elevation of the erythrocyte sedimentation rate in the elderly warrants investigation for the presence of a monoclonal gammopathy. Agarose gel electrophoresis and immunofixation identify a higher percent of monoclonal gammopathies in the elderly than has previously been recognized. Identification of monoclonal components in this population is useful for the subsequent study of plasma cell dyscrasias, neoplastic disease, or other immune dysfunction in the aged.