A relation between antigenic properties and virulence of Leishmania clones were studied. The clones were obtained from a naturally infected sandfly (Phlebotomus of the group caucasicus) caught in the burrow of great gerbil in Turkmenia. Antigenic properties of the clones were studied by means of Adler's test (Safjanova's modification); the virulence of the clones was studied on golden hamsters by means of standard technique. There was revealed heterogenicity of the Phlebotomus strain of Leishmania, in it there were found clones identical serologically to Leishmania donovani and L. major and a clone close in antigenic respect to Leptomonas pessoai. At the experimental infection of hamsters with a heterogeneous Phlebotomus strain their mixed infection with the two above mentioned Leishmania species was first observed. Serologically identical Leishmania clones can differ essentially in their virulence. A high stability of antigenic properties of Leishmania clones has been revealed. At the same time the virulence of clones is characterized by variability as towards its increase (the passing of a clone on hamsters) so towards its fall (cultivation of a clone in vitro, on nutrient medium).