Level of alpha 1-proteinase inhibitor (alpha 1-Pi) and antitryptic activity in blood serum were assessed in 167 patients with breast cancer and 30 cases of benign lesions. An increase in blood serum-alpha 1-Pi level was shown to be associated with tumor advancement but could not be regarded as a marker of cancer. Concentration of the inhibitor exceeded 8 mg/ml in 83.4% of cases with metastatic breast cancer but was as a rule, equal to or below that value in metastasis--free patients. Those two groups were prognostically different. Morphological analysis established a correlation between blood serum-alpha 1-Pi level, on the one hand, and grade of malignancy, degree of pathologic changes in tumor stroma and extent of lymph node involvement, on the other.