Abstract: The present invention discloses methods for isolating, proliferating and differentiating germ-line stem cell in vitro. More specifically, the present invention discloses a method for the in vitro isolation and proliferation of mammalian germ-line stem cells, characterized in culturing cells isolated from mammalian testis in an embryonic stem cell culture medium, and for the in vitro differentiation, characterized in encapsulating mammalian germ-line stem cells and Sertoli cells with calcium alginate and co-culturing with peritubular cells. Also, the present invention discloses germ-line stem cells obtained by above methods, a composition for the treatment of male infertility, comprising the germ-line stem cells, and a method for the treatment of male infertility by the use of the germ-line stem cells.