Thermo-efficient and thermo-sensitive mutants of pox viruses are obtained, without the aid of mutagens, only with difficulty. Kirn and his colleagues (1, 2) have described both a thermo-sensitive and a thermo-efficient mutant of vaccinia virus. This note reports the derivation and properties of two thermo-efficient strains of variola major virus.

The first strain () was adapted by serial passage in the chick chorioallantois (CAM) at temperatures which were increased from 37° to 39.8° in small steps as adaptation occurred. After 44 passes a clonal stock (t) was prepared. The other strain () was adapted by a similar process except that occasional passes at 35° were made to increase the titre of the passage virus. This enabled bigger incremental steps to be made in the growth temperature than was possible when using continued passages at elevated temperature. After 22 passes a clonal stock (t) was prepared.

. Suspensions containing 10 pock forming units (pk.f.u.) per ml. were prepared from the stocks of parent and adapted viruses and titrated with appropriate dilution on the chorioallantoic membrane (CAM) at different temperatures of incubation (Table 1).


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