Vitamins A, E and K were incorporated into turkey starter rations at normal (1 x ), 5 x and 50 x the National Research Council recommended vitamin levels and evaluated for their effect on histomoniasis in turkeys. Infected as well as comparable unifected poults were included in all trials. Each vitamin was also evaluated at normal and 50 x the NRC level in combination with ipronidazole, a low-level antihistomonal agent. In addition to performance parameters, glutamate oxaloacetate transaminase (GOT) and lactate dehydrogenase (LDH) plasma enzyme levels were used to evaluate the vitamin effect. None of the vitamins tested alone were found to be effective in terms of reducing mortality and morbidity following a severe Histomonas meleagridis challenge, regardless of the vitamin dose. A significant improvement in performance was demonstrated with the vitamin E (50 X)--ipronidazole combination and a numerical improvement with the vitamin A (50 X)--ipronidazole combination over ipronidazole alone or the poults fed NRC recommended vitamin levels. The GOT and LDH plasma enzyme levels correlated well with the progressive pathological changes associated with the disease. At 11 days post-infection both enzymes were consistently and significantly elevated above those of the uninfected controls, irrespective of the vitamin levels provided. The presence of low ipronidazole in most instances delayed the onset of clinical symptoms and was associated with a delay in enzyme elevation, indicating the excellent correlation and sensitivity of the enzyme method in assessing liver damage.