On the basis of genuine mouse monoclonal antibody ICO25 the test system IEA ICO25 was developed and standardized to quantitative detect tumor-associated antigen, mucin1 in human blood serum in format of inhibitory immune-enzyme analysis. The analytic characteristics of test-system correspond to the standards applied to immune-enzyme diagnostic kits. The results of identification of MUC1 in blood serum of healthy donors and female patients with breast pathology using IEA ICO25 fully correlate with the data concerning the detection of antigen CA15-3 using certified commercial kits. The test system IEA ICO25 can be used to detect MUC1 in human blood serum for research purpose.