The development of vitamin K deficiency, vitamin K requirement and warfarin susceptibility were studied in several types of warfarin-resistant and non-resistant rat. Domesticated HW strain rats needed about 13 times as much vitamin K1 as did Wistar rats to maintain normal blood-clotting function, while HS strain rats were intermediate in this and in their susceptibility to warfarin. The vitamin K requirement of Wistar hybrids resembled that of the Wistar parent or was intermediate. Hypoprothrom-binaemia was induced at least as readily in non-resistant wild rats as in HS rats. A new diet suitable for inducing vitamin K deficiency in conventionally maintained Wistar rats is described, and it is suggested that the HW strain could be used for vitamin K bioassay. The genetical basis of warfarin resistance is discussed.