A new assay based on using an original chromogenic substrate (z-Ala-Ala-Arg-pNa) and designed for the determination of antithrombin III is described in the paper. It was used in examinations of patients with malignancy after surgery. The content of antithrombin III was detected to be lower in 12% of cases in 2 days after surgery. Additionally, patients with DIC, as complications after surgery, were examined to find out that the antithrombin III content was decreasing in them.