Soluble forms of keratins in human sera seem to be useful analytes for the monitoring of cancer patients. CYFRA 21-1 is a new test measuring keratin 19 in human blood. The test was developed as sandwich ELISA based on two monoclonal antibodies, both reacting with keratin 19. The epitopes recognised by the antibodies are located on the rod region of the molecule. In sera from malignant patients CYFRA 21-1 detects immunoreactive compounds which appear to be larger than keratin 19 itself, indicating the presence of oligomers in these sera. In a comparison study the reactivity of other keratin tests (TPA, TPS, TPAcyk) with the keratins 8, 18 and 19 were measured both in solution and on immunoblots. The reactivity pattern was found to be different what may explain the different diagnostic properties of the individual tests.