Nineteen patients with metastatic breast cancer refractory to conventional therapy were treated with plasma perfusion over 200 mg of staphylococcal Protein A immobilized on a silica matrix. Fever and chills (33%), pain at the site of tumor (18%), and dyspnea (16%) were the most frequent toxic effects encountered. Four patients (21%) developed a disseminated rash which necessitated cessation of treatment. Of 16 patients evaluable for response, one achieved a minor response of chest wall disease and two had no change in hepatic metastases for 4 and 5 months' duration. Potential mechanisms of antitumor effect are discussed.