The therapeutic effect of absorbing plasma from tumour patients with immobilized staphylococcal protein A was tested. Plasma prepared by centrifugation was passed over protein A-Sepharose and then reinfused into the patient. Five patients were thus treated. One with malignant melanoma and one with renal adenocarcinoma showed measurable regression of metastatic lesions. In another with malignant melanoma a subcutaneous metastasis showed histopathological changes compatible with a therapeutic effect. In two patients, one with malignant melanoma and one with renal adenocarcinoma, no signs of regression were found. No severe adverse effects of the treatment were observed.