In suspended secondary calf kidney cells infected with foot-and-mouth disease virus (FMDV) the temperature range for optimal virus growth is shifted down by 3 to 5 degrees C in the presence of 1--2 mM guanidine. For some virus strains this shift is so effective that at infraoptimal temperatures virus yield in guanidine-treated cells exceeds that of the corresponding control by more than one log10. On the contrary, at supraoptimal temperatures inhibition of virus growth by the drug is strongly enhanced. At a concentration of 1 to 2 mM guanidine virus yield reduction or enhancement is based on a decrease in increase, respectively, of the number of virus producing cells (infective centers; I.C.), while virus yield per I.C. is less affected. Besides this "thermomimetic" effect virus production is inhibited by guanidine depending on the concentration of this substance. A mutant of FMDV strain O1L, resistant to 4.2 guanidine, did not differ from the original virus in its antigenic behaviour in the passive immunohemolysis test.