Abstract: The invention concerns the transformation of human T-lymphocytes to stable growth by infecting said T-cells with strains of H. saimiri subgroup NonA NonB, preferably subgroup C. The transformed cells grow independently of antigen stimulation and can have a phenotype of activated mature T-lymphocytes, expressing cell surface markers characteristic for the distinct differentiation status of the T-cells. Infection with strains of H. saimiri belonging to the subgroups above can thus be used to generate human T-cell lines for a variety of immunological and biochemical applications as well as for therapeutic use.