Young New Hampshire and broiler chickens were given feed containing 2 concentrations of aflatoxin B1 (0.2 or 2.0 ppm). The larger concentration caused severe toxicosis and death in the New Hampshire chicks, but did not cause gross signs in the broiler chicks. However, temporary stunting was seen on both New Hampshire and broiler chicks during periods of aflatoxin feeding, along with persisting decreased weight gains in the broiler chicks; the latter apparently recovered during the next 21 days of feeding the starter ration. New Hampshire and broiler chicks which were given feed containing aflatoxin B1 at concentrations of 0.2 and 2.0 ppm for 28 days followed by a 21-day "recovery period" and which were not given a coccidiostat were more susceptible to severe cecal coccidiosis and had more persisting hepatic and cecal lesions than did chicks not given aflatoxin. The coccidiostat was protective against both cecal damage and losses in checks challenge exposed at 49 days of age to 100,000 infective Eimeria tenella oocysts.