The factor VIII:C-sensitivity of the aPTT in dogs was investigated for 2 different, commercially available reagents. For this purpose, 45 plasmas with a decreased factor VIII:C-activity, which were collected from haemophilic dogs (n = 10) at variable points of time of a substitution therapy were used. Moreover, by the stepwise addition of a canine factor VIII:C-deficient plasma to a reference pool plasma it was examined whether factor VIII:C-sensitivity could be increased by prediluting the plasma. By use of both reagents all 45 pathological plasmas (factor VIII:C-activity: 1.5-58%, reference values: 63-144%) showed a prolonged aPTT in relation to the respective reference values. The aPTT correlated well with a pathologically decreased factor VIII:C-activity. In conclusion, the aPTT measured with commercially available reagents according to the manufacturers' instruction is a well suited screening test for haemophilia A and monitoring of its therapy in dogs. Predilution of the dog plasma used in the test does not lead to a distinct of the sensitivity. On the contrary, a high dilution leads to a decrease of sensitivity.