An in vitro and in vivo comparison of nine commercial and noncommercial factor VIII preparations was made. These consisted of one lyophilized cryoprecipitate, four intermediate (IPC) and four high purity concentrates (HPC). Protein, fibrinogen, factor VIII complex, IgG, IgM and anti-A and B alloagglutinins levels were measured. These three qualities of product were defined by two ratios: units of F VIII : C per mg of protein and per mg of fibrinogen. They were, respectively, <0.5 and <1 in cryoprecipitate, 0.5-1 and 1-3 in IPC, and >1 and >3 in HPC. The F VIII : C/F VIII : AG ratio ranged from 0.3 to 0.6 and the F VIII : C/F VIII : VWF ratio was always lower than 1. Varying titers of alloagglutinins were found, unrelated to either IgG or IgM levels. Seven of these preparations were injected into several classical hemophilia A patients for treatment of minor hemorrhages. The peak of F VIII : C activity was always found 1 h postinjection. The F VIII : C recovery ranged from 80 to 140% and the half-life from 8 to 15 h. No significant difference was found among these products and the clinical efficacy was similar.