We investigated the ability of young lambs to develop protective immunity to gastro-intestinal nematode following immunisation with drug-abbreviated infection. Thirty, 3-4 month old lambs were randomly allocated to five groups. Groups 1, 2 and 3 were immunised by three Oxfendazole-abbreviated artificial infections of Trichostrongylus colubriformis and Ostertagia circumcincta. Group 1 received three high immunising larval doses. Groups 2 and 3 were immunised in the same way as Group 1 but with 50 and 75% of the larval dose, respectively. Group 4 was treated with Oxfendazole at the same time as Groups 1, 2 and 3, while Group 5 was an untreated control. All groups grazed the same pasture. At the end of experiment the lambs from Groups 1 and 4 had higher liveweight gains and a higher dag weights than Group 5. Mean total faecal worm egg count was reduced in animals from Groups 1-4, compared controls. However, because of the limited efficacy of vaccination of 3-4 month old lambs by drug-abbreviated infections we suggest that immunisation of young lambs may not be practical unless accompanied by appropriate immunostimulation of the gut mucosa.