Thorne, H. V. (Research Institute, Animal Virus Diseases, Pirbright, England). Kinetics of cell infection and penetration by the virus of foot-and-mouth disease. J. Bacteriol. 84:929–942. 1962.—The kinetics of cell infection by the virus of foot-and-mouth disease were determined, using acidification to pH 5.7 to eliminate virus from the system and to stop the reaction at required times. The infection curve was sigmoidal in form with an initial lag stage followed by a linear region of slope, at moderate concentrations, proportional to the virus and cell concentrations. The infection rate was very small below 15 C but increased rapidly with increasing temperature. Cell infection was 90% complete in about 3 min at 37 C and about 20 min at 25 C. By comparison with kinetic equations derived for a simple model of the infection process, adsorption and penetration rate constants were calculated. The adsorption rate constants were of the same order as those obtained from direct measurements of virus adsorption. The half-time for the penetration reaction was about 30 sec at 37 C and 6 min at 20 C; the rate decreased to near zero values below 15 C. The energy of activation for penetration was 24,000 cal/mole.