Two studies were conducted, one in Argentina and one in Brazil, to evaluate the therapeutic efficacy of doramectin administered subcutaneously at a dose rate of 200 micrograms kg-1 (1 ml per 50 kg) to cattle harboring mixed field infections of gastrointestinal nematodes. In each trial, 20 heifers were randomly allocated to a control or treated group of ten animals each, on the basis of fecal egg counts. Animals in the treated and control groups received an injection of doramectin or saline in the lateral midline of the neck. From 14 to 18 days post-treatment an equal number of animals from each group were slaughtered every day and the worm burdens of doramectin-treated animals were compared with those of non-treated controls. The efficacy of doramectin was at least 99.9% against adult stages of Ostertagia ostertagi, Haemonchus placei, Haemonchus contortus, Haemonchus similis, Trichostrongylus axei, Cooperia pectinata, Cooperia spatulata, Cooperia surnabada (Cooperia mcmasteri), Oesophagostomum radiatum and Dictyocaulus viviparus. Efficacy against Nematodirus helvetianus was 97.9% while efficacy against Trichuris discolor was 92.3%. Activity against inhibited larval forms of Ostertagia ostertagi, H. placei, Oesophagostomum radiatum, and Trichostrongylus axei, was at least 99.9%.