There are two forms of factor VIII (F VIII), the high molecular weight factor VIII (HMW F VIII) and the low molecular weight factor VIII (LMW F VIII), in rabbit plasma. Rabbit LMW F VIII was adsorbed by Al(OH)3 and did not appear in the cryoprecipitate. By utilizing adsorption with Al(OH)3, elution with 0.15M sodium citrate and concentration with polyethylene glycol, we isolated rabbit LMW F VIII from rabbit plasma. Finding that rabbit LMW F VIII could be adsorbed by Al(OH)3, we obtained a procoagulant by utilizing this on the assumption that such a LMW F VIII was present in a small quantity in human plasma. This procoagulant activity eluted at the same elution volume where rabbit LMW F VIII eluted on the 6% agarose gel column and was inhibited by rabbit antihuman F VIII serum, indicating that it was human LMW F VIII. Rabbit LMW F VIII did not combine with rabbit HMW F VIII subcomponent which dissociated from purified HMW F VIII in the presence of 0.25M CaCl2. Also, human HMW F VIII subcomponent which dissociated from purified HMW F VIII in the presence of 0.25M CaCl2 did not combine with human LMW F VIII isolated by above method. These results indicated that LMW F VIII was also present in human plasma and that it had properties some what different from those of the LMW F VIII subcomponent dissociated from human purified F VIII under high ionic strength.
