A possible horizontal gene transfer from transgenic plants by natural transformation to the soil bacterium Acinetobacter calcoaceticus BD413 is investigated through measurement of the transformation frequencies with DNA from two sources of transgenic plant DNA and different forms of plasmid DNA with an inserted kanamycin resistance gene, nptII. Clear effects of homology were seen on transformation frequencies, and no transformants were ever detected after using transgenic plant DNA. This implied a transformation frequency of less than 10-13 (transformants per recipient) under optimised conditions, which is expected to drop even further to a minimum of 10-16 due to soil conditions and a lowered concentration of DNA available to cells. Previous studies have shown that chromosomal DNA released to soil is only available to A. calcoaceticus for limited period of time and that A. calcoaceticus does not maintain detectable competence in soil. Taken together, these results suggest that A. calcoaceticus does not take up non-homologous plant DNA at appreciable frequencies under natural conditions.