This study investigated the effects on growth and composition of the blood of dietary aflatoxin fed at levels of 0, 2.5, or 5.0 microgram/g of diet for a three week period beginning at 1, 7, 14, and 21 days of age in commercial broilers. Packed cell volume (PCV), erythrocyte counts, mean corpuscular volume, hemoglobin content and mean corpuscular hemoglobin concentration, body weight, and mortality were measured weekly. Plasma cholesterol and total plasma protein levels were determined weekly. Dietary aflatoxin at levels of 2.5 and 5.0 microgram of aflatoxin per gram of diet when fed to young chicks for three weeks, beginning at either one or seven days, depressed body weight and PCV in a dose related fashion. Body weight and PCV continued to be depressed by the 5.0 microgram/g diets in chicks treated from 2 to 5 weeks of age but not at the lower dosages. The feeding of aflatoxin at levels of up to 5.0 microgram aflatoxin per gram of diet from 3 to 6 weeks of age did not significantly depress body weight or PCV from control levels. Plasma cholesterol and total protein were found to be more sensitive to aflatoxin treatment at the later ages than body weight and other blood values. Plasma cholesterol was significantly depressed by the 5.0 microgram/g level of aflatoxin in all treatment periods, but the 2.5 mirogram/g level of aflatoxin did not significantly reduce cholesterol during the 3 to 6 week treatment period. Plasma proteins were found to be the most sensitive criteria for detecting broiler susceptibility to aflatoxin, being depressed by all levels of aflatoxin for all age groups.