Abstract: A new method of controlling worm burden in ruminants by using two different injectable anthelmintics at or about the same time to increase the time between treatments and to rely on the animal's immune response. One of the anthelmintics is a long acting formulation capable of providing a sustained release of abamectin or other macrocyclic lactone for 2-14 days, to enable the anthelmintic to remove or kill incoming larvae for a sufficiently long period of time to allow the host to recognise the incoming larvae as foreign, and thus allow the ruminant to mount an immune response against these incoming larvae. The other is a short acting anthelmintic such as levamisole designed to substantially eliminate the initial parasite population of adults, juveniles and larvae. They can be combined into a single injectable formulation with a high percentage of levamisole (preferably over 30% of the formulation w/v) together with about 1% w/v of a macrocyclic lactone such as abamectin, dissolved in glycerol formal.