Morphologic studies were carried out on the liver, kidneys, and small intestine of broiler birds that had been given antibiotics with the feed (virginiamycin, 20.0 ppm, flavomycin, 4.8 ppm, and avotan, 10.0 ppm) in the course of 49 days, kept with a group of controls. The liver of the test birds showed protein and fatty dystrophy, and the kidneys--protein dystrophy. The small intestine showed thinning of the wall and increase in the villi intestinales length. The manifestation of the morphologic changes depended on the amount of the antibiotic taken in. Those of the birds that were offered flavomycin had well manifested lesions, while in birds that were given virginiamycin and avotan only the lesions were more slightly expressed.