Novel microsphere compositions for use in parenteral formulations are provided. The microspheres comprise a biodegradable polymer of a molecular weight greater than 10, 000 daltons, an active therapeutic agent, and a cellulose-derived material such as ethyl cellulose, carboxymethyl cellulose, hydroxypropylmethyl cellulose, or sodium carboxymethyl cellulose. The microsphere compositions decreased deviation in mean microsphere diameter, improved drug entrapment, and improved microsphere stability.