Regeneration from apple (Malus × domestica Borkh.) M.26 leaf tissue was completely inhibited by (μg·ml-1) 1 geneticin, 5 kanamycin, 10 to 25 paromomycin, and 100 neomycin. nptII-transgenic M.26 had an increased tolerance to all four of the antibiotics tested, with inhibition of regeneration occurring at (μg·ml-l) 2.5 geneticin, 100 kanamycin, 375 paromomycin, and 375 neomycin. Paromomycin (100 to 250 μg·ml-l) and neomycin (250 μg·ml-1) significantly increased the amount of regeneration from nptII-transgenic M.26 apple leaf tissue. p35SGUS-INT, a plasmid with a chimeric b -glucuronidase gene containing a plant intron, was useful for studying the early events of apple transformation by eliminating GUS expression from Agrobacterium tumefaciens. It was used to determine that the optimal aminoglycoside concentrations for the selection of nptII-transgenic M.26 cells were (μg·ml-1) 2.5 to 16 kanamycin, 63 to 100 neomycin, and 25 to 63 paromomycin. Geneticin was unsuitable as a selective agent.