Variations of the partial thromboplastin time (PTT) were tested to determine the best screening method for detection of inhibitors of factor VIII. Variables tested included the duration of preincubation of a mixture of patient plasma and factor VIII source (normal plasma), the ratio of the patient plasma to the normal plasma, and the duration of incubation of the normal plasma-patient plasma mixture with kaolin-cephalin suspension prior to recalcification. The following conclusions were reached: (1) The PTT performed on a mixture of equal amounts of patient and normal plasma without preincubation of the mixture was inadequate to detect many factor VIII inhibitors. (2) Factor VIII inhibitors of more than 0.5 Bethesda units could be detected if the PTT was performed on a mixture of four parts patient plasma and one part normal plasma, with preincubation of the mixture for 60 min at 37 degrees C. (3) Factor VIII inhibitors as weak as 0.1 Bethesda units could be detected if the PTT was performed on a mixture of four parts patient plasma and one part normal plasma incubated with kaolin-cephalin suspension for 120 min at 37 degrees C before recalcification. The last method may make detection of mild factor VIII inhibitors possible in routine clinical laboratories not equipped to perform the more technically difficult Bethesda inhibitor assays.