A long-acting, extended-release microsphere formulation of the acetate form of goserelin, a synthetic decapeptide analog of luteinizing hormone-releasing hormone (LHRH), with potential antineoplastic activity. Upon administration, goserelin binds to and activates pituitary gonadotropin-releasing hormone (GnRH) receptors. Prolonged administration of goserelin inhibits the secretion of pituitary gonadotropin, thereby decreasing levels of testosterone (in males) and estradiol (in females). Administration of this agent in an extended-release formulation may result in the regression of sex hormone-sensitive tumors and a reduction in sex organ size and function.