Blocks polypeptide synthesis by inhibiting protein elongation. For use in the selection and maintenance of eukaryotic cells stably transfected with neomycin resistance genes. Blocks protein synthesis by interfering with ribosomal function. Not normally used as a standard antibiotic, but rather for selection and maintenance of prokaryotic and eukaryotic cells stably transfected with neomycin resistance genes. Resistance is conferred by one of two dominant genes of bacterial origin which can be expressed in eukaryotic cells. The resistance genes are APH (3') II or aminoglycoside phosphotransferase 3'(II) and aminoglycoside phosphotranferase 3'(I) or APH (3') I. The following concentrations of active ingredient may be used as a guide for selection: mammalian cells (400 ug/ml), plants (10 ug/ml), yeasts (500 ug/ml) and dictyostelium (10 ug/ml). Cells have variable resistance and multiplying cells will be affected sooner than those that are not. Cells in log phase may require three to seven days for selection. Varying concentrations should be tested. Additionally, fibroblast contamination in mixed cultures may be suppressed by using 100 ug/ml of the active ingredient for 2 days