Sera and synovial fluids from thirty-seven patients with various types of arthropathy were studied for the presence of cryoprecipitates. The isolated, washed cryoprecipitates were analysed for immunoglobulins, antiglobulins, antinuclear antibody and DNA content. Cryoprecipitates were detected in the synovial fluids, but not in the matched serum specimens of twenty-six subjects, twenty-four of whom had rheumatoid arthritis. The precipitates consisted of either IgG—IgM complexes or IgG alone. Antiglobulin activity was present in the cryoprecipitates from nineteen rheumatoid patients; of these nineteen, fifteen had antiglobulins present in serum and fourteen had antiglobulins in their whole synovial fluids. Antinuclear antibody was detected in the cryoprecipitates from ten rheumatoid patients; only three patients had antinuclear antibodies in serum and/or whole synovial fluid. DNA was demonstrated in three cryoprecipitates and seven whole synovial fluids using an antiserum specific for calf thymus single strand DNA. When the cryoprecipitates were heat-denatured prior to testing, seventeen were shown to contain DNA antigens. The synovial fluids from one patient with Reiter's syndrome and one subject with psoriatic arthropathy also formed cryoprecipitates, but neither contained antiglobulin or antinuclear antibody activity. These observations suggest that the cryoprecipitates in rheumatoid synovial fluids contain several different antibodies and antigens, including antiglobulins, immunoglobulins, antinuclear antibody and DNA. It is postulated that these cryoprecipitates may represent cold insoluble antigen—antibody complexes.