Abstract A cryoglobulin precipitate isolated from human blood serum was found to consist of two protein fractions, with sedimentation constants 19.8 S and 6.7 S. Both fractions are immunologically and electrophoretically related to γ-globulins (19.8 S to γM and 6.7 S to γ g). Amino acid analysis confirms this relationship, though there are some deviations concerning especially the 19.8 S fraction. Each fraction separately is easily soluble, but precipitates after mixing. Fraction 19.8 S, added to γ g or γ m-globulin from normal serum, produces also the precipitate. Solubility of precipitate depends on pH, salt concentration and temperature. Temperature strongly influences the precipitation in the range between 23° and 37°. Below 23°, the temperature has hardly any influence on the precipitation.