The serum proteins of 48 patients with adenocarcinoma of the breast were studied by electrophoresis, with particular attention to the α2-globulin fraction, and compared with sera of 162 apparently healthy persons. The α2-globulin fraction was elevated in 33 of 50 patients (in one of 11 with local disease and in 32 of 39 with spread beyond local nodes). The serum albumin level was depressed in 32 of 50 patients (two of 11 with local disease and 30 of 39 with spread). The α2-globulin elevations correlated well with bone metastases. The evidence suggests that α2-globulin levels are valuable indicators of distant metastases in breast cancer.