The progression of aflatoxicosis was evaluated in growing crossbred barrows given 0, 1, 2, 3, or 4 mg of aflatoxin (AF)/kg of feed for 28 days (6 to 10 weeks of age). On day 28, pigs were euthanatized and necropsied, and tissues were removed for histologic examination. Body weight gains were decreased in barrows fed 2 mg of AF/kg after 7 days and in barrows fed 1 mg of AF/Kg after 14 days. By 28 days, all barrows fed AF had decreased body weights and weight gains. Compared with decreased in all barrows fed AF. Neither liver weights nor bone ash values were altered, although liver lipid values were increased in barrows fed AF. Serum aspartate transaminase, gamma-glutamyl transferase, and alkaline phosphatase activities were increased in barrows fed AF, whereas creatine kinase activity was decreased. Aflatoxin diets resulted in decreases in serum concentrations of urea nitrogen, phosphorus, cholesterol, albumin, and total protein. Histologic alterations in liver included interlobular fibrosis, periportal lipidosis, bile duct hyperplasia, and periportal lymphocytic infiltration. Lymphocytes in the thymus were depleted, and numbers of granulocytic cells in the bone marrow were reduced. The frequency and severity of lesions increased with increased doses of AF.