Abstract.Cryoglobulin precipitates of the mixed type, containing 19 S IgM and 7 S IgG gamma globulin, were isolated from a patient serum and the interaction with serum C was studied. Loss of C activity in human serum incubated with cryoprecipitate was accompanied by the generation of C3 activity in the cryoglobulin aggregates. Complete dissociation of cryoprecipitation and the interaction of the precipitates with C was achieved at temperatures below 22°C and above 32°C respectively. The normal C titre in fresh serum of a cryoglobulinaemic patient was significantly reducedin vitroby first cooling and then rewarming the serum. The possible pathogenic role of the bithermic C activation in cryoglobulinaemic vascular lesions is discusse