SummaryFactor Vlll‐related antigen (F Vlll:AGN) in 21 canine plasma samples was assayed by immunoelectrophoresis using a rabbit anti‐canine F VIII serum prepared from a canine F VIII concentrate and a commercial rabbit antihuman F VIII serum. A good correlation existed (r value 0.916) between the antigen levels obtained using the two sera.In normal dogs the plasma FVIII. AG/V level was 95 ± 39% (Mean ± SD) compared to 175 ± 40% in dogs with severe hemophilia A and 17 ± 15% in dogs with von Willebrand's disease.It was concluded that there was sufficient cross reactivity between canine F VIII and commercial rabbit antihuman F VIII serum to make the latter useful in the differential diagnosis of F VIII deficiencies in