Serum cryoglobulins were found in nineteen out of twenty patients with infectious endocarditis. The cryoglobulins were of the 'mixed type' consisting of IgG, IgM and IgA. C3 and fibrinogen were present in some specimens. The concentration of the cryoglobulins tended to fall with therapy and clinical improvement of the patients. Serum antibodies to the offending bacterial organism were not preferentially concentrated in the cryoglobulins. In contrast, IgM rheumatoid factor was present in the cryoglobulins, though undetectable in the corresponding serum. These findings are consistent with the view that cryoglobulins represent circulating immune complexes which may be important in the pathogenesis of immunological sequelae sometimes found in patients with infectious endocarditis.