Free light chains of immunoglobulins were measured in serum from 181 patients with rheumatoid arthritis (RA), sarcoidosis, pulmonary tuberculosis, pulmonary cancer and chronic bronchitis, i.e. patients with long-term stimulation of the immune system. Increased concentrations of light chains were found in patients with active sarcoidosis, and the light chain level appeared to be a marker of disease activity in sarcoidosis. The mean (+/- S.D.) concentration of kappa plus lambda chains in active sarcoidosis of more than two years' duration was 38.3 +/- 13.7 mg/l, that is twice the concentration of 19.4 +/- 5.5 mg/l found in normals. Some patients with seropositive RA also had increased serum values. The light chain concentrations in seropositive RA were correlated to the severity of the disease measured by clinical staging and laboratory tests. A modest increase in light chains was observed in one patient with tuberculosis, and in two patients each with chronic bronchitis and pulmonary cancer.