Guinea pigs, weighing 250-350 g, were infected with approximately 5,000 of live germs M- tuberculosis H 37 Rv grown 10 days in deep culture of liquid semisynthetic medium according to Sula. The infection was performed subcutaneously in inquinal region. For the therapy following phages were used: DS-6A, GR-21/T, My-327 injected twice a week subcutaneously in the dose of 10(6)/1 ml of live particles for 10 weeks. The therapeutic effect was expressed by spleen and hilus index. Out of the phages used, phage DS-6A had the highest therapeutic effect with mean spleen index of 0.19, corresponding approximately to the spleen index reached with the most effective tuberculostaticum INH. The exact explanation of the phage therapeutic effect in given experimental conditions, when the phages are not applied locally in order to gain the direct contact with infectious antigens, is not known. It is suggested that there presumably exists an interaction between the released phage nucleic acid and the nucleic acid synthesis needed for the growth of mycobacteria in vivo.