A chick tracheal-organ-culture model system was used to evaluate the sensitivity of various strains of chickens to aflatoxin B1. Tissues from different genetic stocks showed highly significant differences in in vitro sensitivity to aflatoxin. Tracheal tissues from brown-egg layers were most sensitive, meat-type birds intermediate, and white-egg layers most resistant. Within-category strains and sex differed significantly in sensitivity to damage by aflatoxin, with male chicks more resistant than female chicks.