The differential susceptibilities of warfarin-susceptible LAC-grey and warfarin-resistant HC house mice to the anticoagulant effect of the oral rodenticide 3-(alpha-acetonyl benzyl)-4-hydroxycoumarin (Warfarin) in terms of their blood clotting times, were determined. The hypoprothrombinaemic effect of both the R(+) and S(-) warfarin enantiomers was also investigated, in addition to the standard test for warfarin-resistance in mice, the ability to survive on a diet containing 0.025% warfarin for 21 days. Onto this base of knowledge of the exact hypoprothrombinaemic responses evoked by treatment of both warfarin- susceptible and warfarin-resistant mice with warfarin at various doses, a structured analysis of the biochemical consequence(s) of expression of the major warfarin-resistance gene, War, could be built. Thus, changes in the in vivo pharmacokinetic parameters including half-life (t[1/2]), plasma clearance (Cl[p]), apparent volume of distribution (Vd[app]) and bioavailability (F) were documented for both R(+) and S(-) warfarin in both males and females of the two mouse strains. Similarly, in vitro hepatic microsomal metabolite profiles following pretreatment with warfarin, phenobarbitone, beta-naphthoflavone and clofibrate, excretion of unchanged warfarin enantiomers and warfarin metabolites and finally plasma protein binding parameters were determined in LAC-grey and HC mice. Therefore, it was possible to correlate changes in the pharmacokinetics, metabolism and disposition of warfarin in these mice with their differential anticoagulant sensitivities. Accordingly, the biochemical mechanism(s) of the expression of the major warfarin-resistance gene, War, has (have) been proposed to be due, at least in part, to a combination of a greater plasma clearance of the more potent S(-) warfarin enantiomer in females, a larger hepatic uptake of the same enantiomer in both sexes, and a greater degree of plasma protein binding of both enantiomers of warfarin.