To evaluate the role of fibronectin (Fn) in factor VIII (FVIII) and von Willebrand factor (vWf) cryoprecipitation, factor VIII procoagulant activity, factor VIII coagulant antigen, factor VIII-related antigen and von Willebrand ristocetin cofactor activity were measured in cryoprecipitate and cryosupernatant from normal and Fn-depleted plasmas. Following cryoprecipitation of normal plasmas, most of the FVIII and almost all the FvWf recovered were found with a part of Fn and of fibrinogen in cryoprecipitate. Fn-depleted plasmas prepared either by affinity chromatography on gelatin or by immunoadsorption on monoclonal anti-Fn antibodies behaved differently: although their cryoprecipitate contained normal fibrinogen levels, neither FVIII nor FvWf was precipitated. Experiments performed with Fn-depleted plasma to which purified fibronectin had been added, and samples of plasma with decreased Fn levels (0.01 to 0.2 g/l) suggest that there is a relation between initial Fn level and the extent of FVIII/vWf cryoprecipitation. We conclude that Fn, like fibrinogen, is necessary to induce cryoprecipitation of FVIII/vWf and that an initial plasma level of 0.2 g/l is sufficient to obtain good recovery of FVIII/vWf in cryoprecipitate.