Moniliformin (1-hydroxy-cyclobut-1-ene-3,4-dione), either purified (0, 8, and 16 mg/kg of diet) or from culture of Fusarium moniliforme strain NRRL 6322 on corn grits (8, 16, and 64 mg/kg of diet) was fed to growing broiler chicks from 1 to 21 days of age. Up to 16 mg moniliformin/kg of diet from either source was without effect on chick weight gain, feed consumption, and mortality. Chicks fed 64 mg moniliformin/kg of diet from culture had reduced weight gain and feed consumption. Total daily moniliformin consumption by these chicks was nearly twice the reported single oral 50% lethal dose. Three of 10 chicks fed 64 mg/kg of moniliformin in the diet died. No lesions were found upon necropsy. The LD50 of purified moniliformin upon intravenous injection of 7-week-old female broiler chickens was 1.38 +/- .035 mg/kg body weight. Average time to death was 65 minutes. Progressive symptoms noted included lack of muscular coordination, tachypnea from moderate to severe followed by slow labored respiration, coma, terminal agonal struggle, and death.