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Subtype antigenic differences exist between strains of the virus of foot-andmouth disease (FMD) within the seven immunologically distinct types. Reports of effects of this diversity were reviewed briefly in a recent paper (Hyslop, Davie & Carter, 1963 passaged serially by inoculating clarified suspensions of epithelium into the mucosa of the tongues of partially immunized cattle. In the early passages a 1/10 suspension was employed, but later the dilution was increased to 1/1000, then to 1/10,000 and finally to 1/100,000. Vesicular material was usually harvested from tongue lesions, but on two occasions epithellum was harvested from secondary lesions on the feet, so that only virus capable of generalizing in the host in the presence of circulating antibody was selected on these occasions.Virus of substrains was propagated also in secondary monolayers of pig kidney cells (PKTC) which were cultured in EYL medium (Earle's saline containing 0.01 % yeast extract and 0 5 % lactalbumin hydrolysate) for 48 hr. before inoculation of the virus.

Suspensions of FMD virus treated with 0·05% formalin at 26° C. for periods up to 144 hr. remained infective for cattle, although the infectivity could not be detected in the presence of aluminium hydroxide. Infectivity was detected in similar virus suspensions which had been treated with 0·05% AEI at 37° C. for 8 hr. but not in suspensions treated for 12 hr.Vaccines prepared from these suspensions were antigenically potent and serum neutralization tests demonstrated the development and regression of serum antibody. The AEI vaccines were at least as potent as the corresponding formalin vaccines.

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Antigenic differences between the strains RV 11 and SA 13/61 of foot-and-mouth disease virus (type SAT 1) were studiedin vivoby cross-protection tests. Cattle inoculated with formolized antigen of either strain developed good immunity to experimental infection with the identical strain but little resistance to the other strain.In vitrothe results of complement fixation tests and of serum-virus neutralization tests in tissue culture were consistent with the observations madein vivo. The results of studies on the serological relationships between four strains of type SAT 1 are presented.The importance of strain differences in the epizootiology and control of the disease is discussed briefly.The authors wish to thank Dr I. A. Galloway and Dr J. B. Brooksby for their advice and criticism, and to acknowledge the valuable technical assistance of Mr K. Herniman, Mr R. L. G. King and Mr E. Scoates.