Cryogloculins were examined in a standardized manner in an unselected group of 35 patients with rheumatoid arthritis (RA) and 8 patients with RA complicated by cutaneous vasuclitis and neuropathy. Optimum conditions for detection and characterization of cryoglobulins were established; the proportion of resolubilized to total precipitable protein remained constant in an individual patient under these conditions. All 8 vascultis patients and 9 of 35 other patients with RA exhibited cryoglobulins; total protein and immunoglobin content were significantly higher in the cryoglobulins of patients with vasculitis. Immunoglobulins G and M constituted two-thirds and three-quarters of the total protein in the cryoglobulins from uncomplicated rheumatoid and vasculitis patients, respectively. Serum antiglobulin titers were higher, and serum C3 levels were lower, in vasculitis patients compared to rheumatoid patients without vasclitis. Anti-gamma globulin activity was detected in all cryoglobulins from vasculitis patients. Cryoglobulin IgG and IgM were polyclonal. Density gradient analyses demonstrated the majority of the cryoglobulin activity to reside in the 19S IgM fraction. There was no evidence of a light weight (8S) IgM. A monoclonal rheumatoid factor did not detect 7S-ANTI-7S complexes in the cryoprecipitates, but acid eluates from some cryoglobulins absorbed with insoluble IgG revealed an antiglobulin of the IgG class. Serial studies performed on vasculitis patients treated with cyclophosphamide disclosed a relationship between clinical evidence of vasculitis and the presence of cryoglobulins. The antigen (IgG) and antibody (largely IgM rheumatoid factor) nature of these cryglobulins is presented as evidence that the widespread vascular complications of RA are mediated, at least in part, by circulating immune complexes.