the present invention comprises novel preparations of polyoxypropylene / polyoxyethylene octablock copolymers which retain the growth - promoting and immunity - enhancing activity of commercially - available preparations , but are free from the undesirable effects which are inherent in the prior art preparations . because the polyoxypropylene / polyoxyethylene copolymers which comprise the present invention have a more homogenous population of molecules with fewer low molecular weight species than prior art preparations , the biological activity of the copolymer is better defined and more predictable . moreover , the polyoxypropylene / polyoxyethylene copolymer of the present invention substantially reduces any risk to human health through the consumption of food animals since the copolymer hereof is not absorbed into the animal &# 39 ; s edible tissue . methods for preparation of the polyoxypropylene / polyoxyethylene copolymer of the present invention are also provided .