Ten parasite free calves were divided into two groups of five. One group was given a prototype intraruminal bolus designed to give a slow release of levamisole. Both groups of calves were then infected with five thousand infective larvae of O ostertagi and C oncophora for six weeks. Eight weeks after the start of infection all the calves were slaughtered and total worm counts carried out. The mean plasma levamisole concentration of treated calves twenty four hours after insertion of the bolus was 0.37 microgram/ml, but rapidly decreased and by two weeks was 0.054 microgram/ml. It remained close to that level for a further three weeks then reduced to 0.02 microgram/ml. The geometric mean worm counts of O ostertagi was reduced by 27.7% and of C oncophora by 100% in treated calves. It was concluded that the circulating concentration of levamisole was sufficient to inhibit O ostertagi for only two weeks but was effective against C oncophora for at least six weeks.