A test for the evaluation of anthelmintics against the larval stages of nematodes is described. Worm-free lambs are given doses of infective larvae of several species of nematode in such a way as to ensure that all the developmental Stages are present on the day of treatment. The drug under test is administered by intraruminal injection on day 0 and controls are slaughtered on day 0 or day 1 and day 3 to ascertain the numbers of worms present at the time of dosing. The treated sheep are killed from days 1 to 3 after dosing. The worms are recovered by Shone's water bath method and counted. Details are given of all the techniques used in the test.