Several tumor cell lines were obtained by transforming Nicotiana tabacum plants with the recombinant Ti plasmid comprising the gene encoding EcoRII DNA methyltransferase (M·EcoRII) and subjected to analysis. The transformed lines differed in their morphology, growth dependence on hormones, and nopaline-synthesizing capacity. Southern blot-hybridization showed that the M·EcoRII gene was present in the cells of all transformed lines. However, genome analysis using polymerase chain reaction with the oligonucleotide primers recognizing 5'-ends of the M·EcoRII gene did not exhibit the full-length copies of the gene. Lower methylation of CpNpG sequences characteristic of all transformed cells could result from the disturbance of one of several plant DNA methyltransferase genes following its homologous recombination with the M·EcoRII gene.