A survey of type O foot and mouth disease (FMD) virus isolates from northern Thailand was undertaken to determine the relationship between field viruses and the vaccine in use, and to gauge the range of antigenic variation among field viruses. Isolates were collected from the two most recent epizootics, 1986-1987 and 1989-1990, and assessed using a two-dimensional neutralisation test to determine their relationship to FMD type O1 Bangkok 1960 (O BKK/60) reference (vaccine challenge) virus. The critical r value for the survey was 0.259 and all isolates tested were found to have an r value considerably greater than this (range 0.66 to 0.80). The results showed close antigenic relationships between the isolates and the reference virus, and indicated a relatively small range of antigenic variation between the isolates.