Interrelationship between the growth-promoting effect of fish solubles and the gut flora of the chick.

I. The effect of dietary fish solubles on the growth and gut weight of conventional and germ-free chicks and of chicks given potentially growth-depressing agents present in the excreta was investigated. 2. Fish solubles significantly increased the growth of conventional but not of germ-free chicks. Oral administration of fresh, but not autoclaved, droppings depressed the growth of chicks hatched germ-frec. Fish solubles partly overcame the depression due to fresh droppings. 3. Streptococcus faecal& or a bacteria-free filtrate prepared from chick droppings did not depress growth when given singly but did when given together. The depression in growth was entirely prevented by fish solubles. 4. Compared with germ-free chicks, thc small intestine was heavier in conventional chicks and in those given fresh but not autoclaved droppings. A bacteria-free filtrate also increased the gut weight; Strep.fuecaZis had the opposite effect. When they were given together the gut weight was not different from that of germ-free chicks.