Infectivity titers of foot-and-mouth disease virus (FMDV) types Asia 1 and 0 were reduced by 4 and 2 log units, respectively, after incubation at 37 degrees C for 12 hrs. The stability of the FMDV RNA genome at 37 degrees C was studied using 32P-labelled virus. The RNA of FMDV type 0 was found to be more stable than that of type Asia 1. Oligo(dT)-cellulose chromatography showed that 21% and 31% of the labelled RNA were bound to the column in the case of types Asia 1 and 0, respectively. Possible correlation between the poly(A) tail length, accessibility of the genome to nucleases and thermostability of the infective virus is discussed. A positive correlation between the thermostability of the genome and general distribution of a particular virus type seems to exist. A stable genome associated with poor virus immunogenicity may be responsible for the prevalence of FMDV type 0 in the nature. The isoelectric focussing of structural proteins isolated from the virus samples incubated at 37 degrees C revealed charge differences in the major immunogen between the two FMDV types. A rapid proteolytic degradation of the viral immunogen and stability of the genome may be responsible for frequent outbreaks of FMD, at least, in the endemic countries.