A simple and convenient method for efficiently establishing 8-azaguanine-resistant mutant leukemia and myeloma cell lines (for example, the T cell lines Jurkat and CCRF-CEM, human myeloid/macrophage-like cell lines HL60 and U937, Burkitt lymphoma line Raji and the human myeloma line RPMI 8226), is described. The method relies on culturing the cell lines in RPMI 1640 medium containing 8-azaguanine and supplemented with 15% heat-inactivated fetal calf serum and large amounts of amino acids and vitamins, and removes the necessity for pretreatment with mutagenic reagents such as ethyl methylsulfonate or X-irradiation. The possibility of obtaining mutant cell lines using the method described here is about 15 times greater than using media without high levels of amino acids and vitamins. Hybridomas produced between mitogen-activated human peripheral blood lymphocytes and an 8-azaguanine-resistant Jurkat mutant cell line (established by this method) were shown to produce soluble T cell-derived macrophage activating factor (MAF)-like material.