For monitoring of recombinant human antithrombin III during cell culture processes and subsequent purification steps a rapid method for quantitative determination was developed. The need for the introduction of this rapid method came from the limited availability of a quantitative enzyme-linked immunosorbent assay (ELISA) and the very time-consuming ELISA procedure. The developed method is based on reversed-phase high-performance liquid chromatography using a C 4 column. The separation by gradient elution using water and acetonitrile takes less than 20 min even when complex samples, such as serum containing cell culture samples, have to be analyzed. Automation and a high sample throughput are possible with this reliable method. If necessary, insulin, transferrin and albumin can also be quantified with minor changes of the elution profile.