Thirteen critical tests (n = 11 horses and 2 ponies) and 4 controlled tests (n = 4 donkeys and 6 horses) were performed to evaluate the activity of the experimental macrocyclic lactone compound F28249-alpha against internal parasites of equids. In the critical tests, activity was determined mainly against the large parasites, but 1 critical test also included benzimidazole-resistant small strongyles. In the controlled tests, evaluation of drug activity included large parasites and stomach worms in all 4 tests and lungworms in 2 tests. The period between treatment and euthanasia was 6 to 9 days for the critical tests and 14, 17, or 52 days for the controlled tests. The compound was administered by stomach tube at dose rates of 1, 2, 3, 3.5, or 4 mg/kg of body weight. In the critical tests, removal at all 5 dose rates was 100% for Gasterophilus nasalis (2nd and 3rd instars), Parascaris equorum (mature), Strongylus vulgaris, and Strongulus edentatus from the gastrointestinal tract. For Gasterophilus intestinalis in the stomach, mean removals of 2nd instars were 88% at the rate of 2 mg/kg and 93% to 100% at rates greater than or equal to 3 mg/kg. For the 3rd instars, mean removals were 7% at 1 mg/kg, 77% at 2 mg/kg, 90% at 3 mg/kg, and 98% at 3.5 mg/kg. Discharge of G intestinalis in feces was typically a slow, prolonged process and probably higher removal values, especially at lower dose rates, would have attended a longer interval after treatment before necropsy examination. There was 100% removal of population B, benzimidazole-resistant small strongyles in the single critical test. Controlled tests at the 4 dose rates between 2 and 4 mg/kg resulted in variable activity against G intestinalis and Oxyuris equi, but efficacious removals were recorded for S vulgaris, S edentatus, Draschia megastoma Habronema muscae, Trichostrongylus axei, and Dictyocaulus arnfieldi. In critical and controlled tests, there was some indication of drug activity against parenteral stages of S edentatus and S vulgaris; activity was best in the control test equids euthanatized 52 days after treatment. There did not appear to be drug activity on eyeworms (Thelazia lacrymalis) or tapeworms (Anoplocephala spp). Toxicosis of transient nature was observed in 1 horse treated at 4 mg/kg.