Recombinant envelope protein E2 (gp55) of classical swine fever virus (CSFV) strain Alfort/187 was evaluated as an alternative to whole virus as ELISA antigen for the detection of antibodies against CSFV. A glycosylated and a non-glycosylated form of E2 was expressed in the baculovirus system. Six histidine residues added at the carboxy terminus of each of the recombinant proteins allowed purification by nickel-chelate affinity chromatography. Comparison of the antigenic properties of the two proteins in indirect and blocking ELISAs revealed that the glycosylated form resulted in both higher sensitivity and specificity. The indirect ELISA, using glycosylated E2, either derived from crude cell extract or affinity-purified, was validated by testing a total of 2719 porcine sera. Its final version proved to be as sensitive (98.3%) as the virus neutralization test when sera from infected pig herds were examined, and highly specific (99.6%) when applied to test negative sera. It is therefore suitable for large scale monitoring of classical swine fever.