A multicenter study evaluated the potential for transmitting non-A/non-B hepatitis, as well as other viruses, with the use of the factor VIIIC product Monoclate. This product is purified from plasma via the monoclonal process that includes heat treatment for ultra-purification as a final step. Twenty different lots of Monoclate were used, and each patient received the assigned lot for the first 6 months of the trial. Nineteen of 38 patients adhered strictly to International Committee on Thrombosis and Hemostasis criteria in that they had normal liver enzymes, no evidence of hepatitis prestudy, and had no previous blood product use. Fourteen hemophilia centers from the United States, the United Kingdom, the Netherlands, and Israel participated in this study. Development of factor VIII inhibitor occurred in six of 38 patients, which was within the statistically expected range. Adverse events were mild, and Monoclate was well tolerated in this group. All 38 patients remained HIV seronegative.