A selection of type O foot-and-mouth disease (FMD) viruses isolated in Thailand between 1986 and 1989 were compared to the reference viruses O1 Thailand 1960 (O BKK/60) and O Nakorn Pathom 1965 (O NPT/65) using a liquid-phase blocking ELISA (LP ELISA) to derive serum titres and associated r values. Interpolation techniques were used to increase the precision for estimation of r values through a more accurate estimation of serum titres at predicted equivalent levels of antigen input. Mean r values were 0.45 (for 56 field viruses) relative to O BKK/60 reference virus and 0.56 (for 51 field viruses) relative to O NPT/65. While only two viruses showed considerable difference (r < 0.20) to a reference virus (O BKK/60), 41% and 31% gave r values less than 0.4 for O BKK/60 and O NPT/65 respectively. This indicated antigenic differences between reference and field viruses which may result in a reduction in vaccine efficacy.