A study of the ability of 49 strains of FMD virus to replicate in BHK-21 monolayer cells maintained under a standard agar overlay containing 5.2 mM guanidine hydrochloride and to withstand heat inactivation at 54 degrees C for 1 h showed that strains belonging to serotypes C, O and Asia 1 were generally more resistant to guanidine and heat stable than the SAT 1, 2 and 3 serotypes. The type A viruses as a whole occupied an intermediate position between these two groups. In vitro passage in BHK-21 cells influenced the guanidine sensitivity of 3(O, C and SAT 3) of the 7 FMD serotypes suggesting that this is not a stable genetic marker. Heat stability of the FMD viruses, however, did not change on passage, suggesting that this is a stable characteristic inherent in any homogeneous FMD virus population.