Antithrombin (AT) is a glycoprotein mainly synthesised in the liver, and is one of the most important inhibitors of coagulation. Although AT activity determination is more important in clinical practice and anti-IIa and anti-Xa chromogenic tests are widely available, sometimes AT antigen measurement is preferred. Immunoradiometric and enzyme linked immunoabsorbent assay (ELISA) tests have been the only methods available so far for AT antigen determination, but both of them are relatively complicated, time consuming, and the ELISA test cannot be used for single sample assessment. A new immunoturbidimetric test (Liatest antithrombin III) is now available. We have adapted it for the Cobas Mira biomedical analyser. The intra- and inter-assay and total coefficients of variation for normal and low control plasma, as well as for normal pool plasma were low (< 3%). Blood samples from 32 patients with different clinical conditions were assessed both by the standard ELISA method and by the immunoturbidimetric method. The correlation coefficient was 0.95. According to our findings it seems that the immunoturbidimetric test (Liatest antithrombin III) for determination of AT antigen automated for the Cobas Mira biochemical analyser is easy to perform, time sparing and precise, with good comparability to other assays. It is suitable for measuring of AT antigen in routine laboratory work.