The development of a blood clotting response test for discriminating between difenacoum-resistant and susceptible Norway rats (Rattus norvegicus, Berk.).

1. A new test for identifying levels of difenacoum resistance in the Norway rat is described, based upon the differential physiological response to difenacoum administration. 2. This test is based on changes in blood clotting activity over 4 days, following administration of the rodenticide difenacoum in conjunction with menadione (vitamin K3). 3. The anticoagulant effect is reduced only in rats that are resistant or tolerant to difenacoum. 4. This test procedure is quicker than traditional feeding tests, and identifies the degree of resistance in both laboratory and wild rats that have difenacoum resistance genes.