The effect of EACA (epsilon aminocaproic acid) on the survival of factor VIII (F. VIII, antihemophilic factor, AHF) concentrates after infusion in hemophilic subjects was studied using two F. VIII preparations: an intermediate purity "cryoglobulin" and a high purity glycine-precipitated concentrate. In vivo recoveries of F. VIII with both concentrates were found to be close to the values predicted on the basis of concentrate potencies assayed prior to infusion, but not if the labeled potency was used in the calculation. Control studies showed similar first and second phase half-life times and half-disappearance times when the two F. VIII preparations were infused, and both preparations were found to follow a biphasic disappearance curve. The biological half-life (second phase rate of disappearance) was 12 to 13 hours. The administration of EACA in conjunction with the F. VIII concentrates did not prolong the half-life times (first or second phase) or the half-disappearance times.