A rapid, qualitative blood test was developed to identify potential bleeders among dogs that are von Willebrand's disease suspects. The test, based on venom coagglu-tinin-induced agglutination of formalin-fixed washed platelets, was compared to the more lengthy electroimmunoassay for factor Will-related antigen. The 2 tests were performed on plasma samples from 82 dogs, including 68 Doberman pinschers. The 39 plasma samples that were found to be “clinically normal” by the new test had factor Vlll-related antigen concentrations ranging from 32% to 192% of that in a pool of normal canine plasma samples. Plasma samples from the 43 dogs that tested as “potential bleeders” by the new test had factor Vlll-related antigen concentrations from 4% to 38%. All 43 “potential bleeders” were Doberman pinschers. Shortly after being evaluated by the new test, 46 of the dogs underwent invasive diagnostic, therapeutic, or cosmetic procedures. Excessive hemorrhage was encountered in 40% of the 25 dogs that tested as “potential bleeders.” In contrast, only 5% of the 21 dogs that tested as “clinically normal” exhibited abnormal bleeding.