The purpose of our study was to compare the therapeutic effects of doxorubicin in 3 different formulations: (1) in PBS, (2) in conventional liposomes composed of egg phosphatidylglycerol/egg phosphatidylcholine/cholesterol/dl-alpha tocopherol, and (3) in sterically stabilized, long-circulating "Stealth" liposomes composed of hydrogenated soy phosphatidylcholine/cholesterol/polyethylene glycol-distearoylphosphatidylethanolamine. The doxorubicin formulations were used to treat recently implanted and well-established, growing primary mouse mammary carcinomas, and to inhibit the development of spontaneous metastases from intra-mammary tumor implants. In the treatment of recently implanted primary tumors, the formulations were given in 3 i.v. injections over 15 days, starting 3 or 10 days after tumor implantation. In the treatment of well-established primary tumors, the mice received 4 i.v. injections over 22 days, starting an average 38 days after tumor implantation. In the preventive treatment against metastases, the formulations were given in 4 i.v. injections over 22 days, starting 22 days or 58 days after primary tumor implantation. The Stealth liposome formulation was significantly more effective than the conventional liposome formulation or the free drug in reducing the incidence of metastases from intra-mammary implants of tumor MC19 and tumor MC65, in curing mice with recent implants of tumor MC2A, tumor MC2B, and tumor MC65, and in increasing the 8-week survival of mice with well-established implants of tumor MC2B. It is concluded that the long circulation time of the Stealth liposome doxorubicin formulation accounts for its superior therapeutic effectiveness.