Diets involving three or five fish meal treatments and two zinc bacitracin treatments were fed to 5410 poults. The poults were divided into 552 pens and raised to two, three or four weeks of age in five series of 3 X 2 or 5 X 2 factorial experiments. Body weights increased 9.0 to 31.3% from the addition of 5% menhaden fish meal and 5.4 to 11.9% from the addition of 44 ppm bacitracin. No interaction existed between fish meal and bacitracin. Residues derived from either water or petroleum ether extraction of fish meal, when added to diets at levels equivalent to those in 5% fish meal, produced body weight gains nearly equal to those obtained from 5% fish meal. Body weights were not increased by the addition of either water or petroleum soluble extracts or by charred or ashed fish meal. The addition of a mixture of crystalline amino acids, defluorinated phosphate, and fat equivalent to 5% fish meal produced only a 4.4% increase in body weight. An unidentified growth factor which is non-lipid, non-mineral, and non-water soluble appears to be present in menhaden fish meal when added to well foritified po