After the subcutaneous injection of type I (alpha) interferon into mice their survival rate in staphylococcal infection greatly increased. At the same time duration of staphylococcal persistence in these animals and the number of persisting staphylococci were found to decrease. After the injection of interferon the splenocytes of the treated animals showed a higher capacity for interferon production. During the whole experiment the characteristics of delayed hypersensitivity in these animals showed a tendency towards normalization in comparison with those in infected mice receiving no interferon.