Jozef Vermylen, * and Jean-Marie Saint-Remy*

Hemophilia A patients producing antibodies towards FVIII are usually treated with infusions of high doses of FVIII in an attempt to “desensitize ” them. To examine the mecha-nisms by which such desensitization operates, sequential plasma samples of two unrelated inhibitor patients were an-alyzed for anti-FVIII and antiidiotypic antibodies before and during infusions of high doses of FVIII. Anti-FVIII an-tibodies were separated from antiidiotypic antibodies by im-munoaffinity chromatography before analysis. We show in the present study that the concentration of anti-FVIII anti-bodies did not change during a successful desensitization and that antibodies maintained their capacity to inhibit the procoagulant function of FVIII, even though the number of Bethesda units in plasma was reduced to undetectable lev