Serum cross neutralisation tests were conducted with a recent American isolate (84KY-A1) and a European isolate, (Wroclaw-2) and compared with the prototype and modified viruses of the Bucyrus strain of equine arteritis virus by using virus specific immune horse sera. The modified Bucyrus strain was neutralised and showed high neutralisation titres with all the immune sera. The prototype Bucyrus strain was also substantially neutralised, followed by the 84KY-A1 strain. As a result of the tests with the modified Bucyrus strain as the antigen, 20 seropositive horses were discovered among home-bred horses which had no previous record of clinical equine viral arteritis. Heat inactivation of the sera at 62 degrees C for 30 minutes caused the disappearance of all but the most positive reactions. When the prototype Bucyrus strain was used instead of the modified virus, no positive reactions were detectable. A serological survey, using the Bucyrus strain in a microtitre neutralisation test, was conducted with 1656 horse sera collected between 1988 and 1990 in Japan. The test disclosed only eight foreign-bred horses positive to the virus; they had been imported as competition horses. No circumstantial evidence of an effect of such horses as a source of infection for horse populations free of the virus was obtained.