Abstract: Disclosed is an animal feed composition that comprises free indole acetic acid (free IAA) or a derivative thereof. Also disclosed is a method for enhancing animal growth by feeding the animal with a composition according to the invention. Further disclosed is the use of free IAA or a derivative thereof in a method of therapy of animals in need of a growth-promoting treatment, such as immunocompromised animals, animals with a growth deficit or slow-growing animals. Disclosed is the use of free IAA or a derivative thereof for the preparation of a therapeutical composition for increasing the growth rate and/or the feed conversion rate and/or the immunity of animals in need of such a treatment, in particular, immunocompromised or slow-growing animals. A composition may be in the form of a food or feed supplement.