The authors present the results of a study designed to compare the immunogenicity of several attenuated strains of sheep pox virus and a virus/immune serum vaccine. Two of the strains studied present immunogenic characteristics that make them particularly interesting for the manufacture of a vaccine. The first, named Djelfa, confers solid immunity to animals without provoking a vaccinal reaction; the second, known as Romania, can provide protection beyond twenty-four months, but causes persistent vaccinal lesions. In a country like Algeria, these two strains could be used for immunoprophylaxis of sheep pox: the first in those regions in which prevalence is low and sheep are vaccinated on a regular basis, and the second in regions of high prevalence where herds are moved to new pastures each season.