The catalase and phosphatase activity of avirulent by keratoconjunctival test of guinea pigs lac+, kcp- hybrids that have lost their ability of penetrating in the epithelium, received from virulent strains Shigella flexneri (Sh. flexneri 222 5a and Sh. flexneri M42-43) and donor strains Escherichia coli (E. coli K-12, P4x and E. coli H) was investigated. It was established that there is no any dependence between the lost of virulence of the hybrids and the level of the catalase activity. During the study of the phosphatase activity it was established, that in comparison with the recipient virulent shigella strains, the investigated avirulent lac+-hybrids revealed higher values of the enzyme activity.