The population of the stock of the Lister strain of vaccinia virus, applied in Israel for immunization against
smallpox, was found to be heterogeneous. A middle-size plaque (MP) virus was found to be the most attenuated for mice,
out of the three variants which we characterized. Previously we found that C-terminal truncation of the A33R viral
envelope protein of vaccinia virus leads to an enhanced release of free virus from the infected cells and to attenuation of
the virus for mice. A recombinant of the MP variant with such truncation, was now constructed and found to release more
progeny virus from the infected cells and slightly more attenuated for mice than its parent virus. This new recombinant of
vaccinia virus may be useful when vaccination against smallpox would be required again, in the future.