In the course of inter-laboratory quality management comparative serological tests on classical swine fever (CSF) are conducted once per year. Results from tests carried out in 1994 and 1995 indicate that most regional diagnostic laboratories were able to classify the test sera correctly as CSF-positive, bovine viral diarrhea (BVD)-positive and negative, respectively. Difficulties were encountered in the differential diagnosis of CSF and BVD in neutralisation tests. There is a need to improve the standardization of CSF serology on the basis of a well established method in order to ensure reliability of test results and to enable comparison of results obtained from different laboratories.