When plasmids carrying leucine genes of Bacillus subtilis 168 were isolated from a restriction and modification deficient (r-m-) strain and used for transformation of a restricting strain B. subtilis 168 leu recE4, the number of transformants was greatly reduced. Transformation of rec+ strain (transformation by integration of the donor DNA into the chromosome) with the plasmids was not affected irrespective of whether the recipient carried the r+ or r- phenotype. These results show that the plasmid-mediated transformation is subject to the host controlled restriction and suggest that r-m- strains should be used for construction of recombinant DNA molecules in B. subtilis 168.