The efficacy of moxidectin, 'injectable' and 'pour on', against gastrointestinal nematodes was determined in cattle in two separate field trials (Trial I in 1990 and Trial II in 1991) with respectively 88 and 94 young grazing cattle of either sex. The efficacy was measured on the basis of the reduction of the egg output and of the evaluation of the results from larval differentiation. Animals in Group MI received 0.2 mg kg-1 body weight (b.w.) moxidectin injectable solution in Trial I on Day 0. Group CI was not given any medication on Day 0, but 0.2 mg kg-1 b.w. ivermectin injectable solution (Ivomec) was applied after 2 weeks to prevent clinical disease. In Trial II, animals in Group MP were treated with pour on moxidectin (0.5 mg kg-1 b.w.) on Day 0. Animals in Group CP serving as controls for Group MP during the first part of the trial received the same formulation at the same dose 2 weeks after treatment of Group MP. When the egg output was compared within treated groups, the egg count reduction was very similar post treatment (p.t.) with both formulations being 96.3% and 96.6% on Day 7 after the application of injectable moxidectin or pour on moxidectin, respectively, and 90.7% and 92.5% on Day 28 p.t. When egg counts of treated and control animals were compared (corrected for the e.p.g. values before treatment) the egg count reduction was 95.4% and 91.5% on Day 7 and 92.9% and 84.8% on Day 14 p.t. with either the injectable or pour on formulation. Pour on moxidectin seemed to be more effective against Ostertagia spp. than against Cooperia spp. Animals treated with injectable moxidectin gained significantly more body weight (4.2 kg per animal) than the controls from Day -7 to Day +14, while no significant difference in weight gain was achieved within 2 weeks after treatment with pour on moxidectin.