1. The assumptions underlying the current methods of determining the efficiency of drugs or a mixture of drugs as an anthelmintic are set out.
2. The method of assessing the efficiency of a given drug or mixture of drugs, as used by Seddon and Ross, is cited and objections to its value are given.
3. A method is suggested whereby the mean efficiency of a drug or mixture of drugs can be assessed, in terms of the initial and residual infestation of selected animals.
4. In the light of the suggested method, Roberts' data for the efficiency of various drugs and mixtures of drugs, as specifics against H. contortus infestation in sheep, are fully investigated.