'Clean’ grazing is accepted as a successful method of overcoming poor performance in lambs and young cattle caused by nematode parasites of the alimentary tract and would virtually eliminate the need for regular anthelmintic treatment. However,it is impracticable on many grass farms and especially where sheep predominate. ‘Safe’ grazing offers a realistic alternative but has not been fully tested under field conditions. Two 2.8 ha paddocks of a predominantly perennial ryegrass sward were managed during the spring and summer of 1985 to produce two different levels of pasture nematode infestation for weaned lamb grazing during the Autumn of that year. Both areas were grazed by non-lactating mature ewes until 5 February 1985. The safe area(S) was then closed for two silage cuts and grazed for 11 days during early August by replacement dairy heifers. The contaminated area(C) was set stocked with 21 cross-bred ewes and their May born Suffolk x lambs per ha between 29 April and 28 August. Dung samples were collected from lambs grazing C at 8,12 and 16 weeks of age. Herbage and dung samples were collected from S and C at weaning (28 August) and 14 day intervals until lambs were removed from trial on 31 October. 138 lambs were divided into two groups by weight and condition at weaning and set stocked at 25/ha on S and C following an anthelmintic treatment. Lambs were selected for slaughter to produce carcasses of MLC fat class 2 or 3.