R adiolabeled human Factor VIII was used to study its survival in normals and patients with classic hemophilia, and to study the heterogeneity of Factor VIII; Purified Factor VIII was radiolabeled with 125iodine (125I-VIII) without loss of its structural integrity. The survival of 125I-VIII was studied in six normals and six hemophiliacs of whom four of the hemophiliacs had received transfusions with normal cryoprecipitate before the 125I-VIII infusion. No significant difference was observed between the disappearance of Factor VIII coagulant activity and radioactivity in these hemophiliacs. 125I-VIII in plasma showed a biphasic disappearance with an average t1/2 of 2.9 +/- 0.4 h (SEM) for the first phase and 18.6 +/- 0.7 h (SEM) for the second phase, respectively. The survival of 125I-VIII was similar comparing normals and hemophiliacs. The highest molecular weight forms of Factor VIII disappear more rapidly than the lower molecular weight ones. This was established by analysis of the fractions obtained by gel chromatography of plasma collected at several times after infusion and by analysis of the in vivo disappearance of three subfractions of Factor VIII. The fraction of 125I-VIII binding to platelets in the presence of ristocetin (containing the highest molecular weight forms of Factor VIII including the ristocetin cofactor) represented about 50% of the radioactivity present in plasma after infusion and showed a t 1/2 of 11.7 +/- 0.9 h (SEM) for the second phase. The fraction, which was recovered in cryoprecipitate of the recipient's plasma, represented about 90% of the initial radioactivity and showed a t 1/2 of 16.3 +/- 0.8 h (SEM) for the second phase. The fraction of 125I-VIII remaining in the cryosupernatant plasma (containing low molecular weight forms of Factor (VIII) showed a t 1/2 of 27.2 +/- 1.1 h (SEM). The first phase of the disappearance of 125I-VIII is caused in part by the disappearance of the highest molecular weight forms, which are possibly removed by the reticuloendothelial system.