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A combination of calciferol (vitamin D2) and warfarin, each at 0·025% in medium oatmeal bait, failed to control six of seven house-mouse (Mus musculus L.) populations infesting urban and farm buildings. In three further treatments with both calciferol and warfarin at 0·05 % in dehusked canary seed bait plus 5% corn oil, mortality, estimated from the consumption of pre- and post-treatment census bait, ranged between 94·2 and 97·4%. Finally, among sixteen treatments done with calciferol at 0·1% and warfarin at 0·025% in various cereal baits, the best results (97·0–100%) were obtained in six treatments where the bait-base was whole canary seed; this was so whether the poison bait was applied directly or after a 3-day pre-baiting period. It is concluded that calciferol at 0·1 % plus warfarin at 0·025 % is an effective combination against house-mice, especially when used with whole canary seed. The role played by warfarin in the poison mixture needs to be investigated further.