The indirect enzyme-linked immunosorbent assay (ELISA) was first developed for the detection of antibodies against BHV-5 in sheep sera. Investigations of various antigen (Ag) preparations revealed that BHV material which had been purified across a sucrose gradient using sonification followed by ultracentrifugation gave satisfactory results during application in ELISA. Optimal coating of the solid phase was achieved with Ag adsorption at 4 degrees C overnight with 5 micrograms of virus material/ml in a carbonate-bicarbonate-buffer, pH 9.7. The background reactions were significantly reduced by blocking the free bonding points with 1% gelafusal after Ag adsorption. As the extinction values of all the sera investigated which a dilution of 1:50 or more below 0.2, the difference threshold between positive and negative sera was set at 0.2. A total of 490 sera from various stocks of sheep were investigated with regard to the occurrence of antibodies against BHV-5 in ELISA. The growing rate of infection with herpesviruses among ruminants and the subsequent economic loss have resulted in the increased significance in the last few years of bovine herpes-virus (BHV) disease.