Mycoplasmas were effectively eliminated from contaminated LTK and CHO cell lines by means of the antibiotic Tiamutin (10 micrograms/ml), while Kanamycin and Gentamycin were less potential. An elimination procedure which involved the consecutive treatment of the cell cloning over a periods of 21-28 days was adopted. This procedure was effective when applied to cell lines contaminated with unidentified and partially non-cultivable strains of mycoplasmas which could be detected by electronic microscope.