the present invention comprises novel preparations of polyoxypropylene / polyoxyethylene copolymers which retain the therapeutic activity of the commercial preparations , but substantially reduce the undesirable effects which are inherent in the prior art preparations . because the preparations of polyoxypropylene / polyoxyethylene copolymers which comprise the present invention are a less polydisperse population of molecules than the prior art polyoxypropylene / polyoxyethylene copolymers , the biological activity of the copolymers is better defined and more predictable and the cardiotoxicity inherent in the native copolymers is substantially reduced .