A random mutation strategy using mutator strain, Epicurian coli XL1-Red, was applied to a plasmid, pND018, constructed by inserting a Lactococcus lacis bacteriophage resistance gene (abil) into a L. lactis/E. coli shuttle vector (pDL278), to introduce random mutations throughout the plasmid. Following transformation of the mutated plasmid library to a plasmid free and phage sensitive strain of L. lactis (LM0230), mutated plasmids were screened by cross-streaking and efficiency of plaquing (EOP) assays. Two strains with enhanced resistance were obtained, as well as several phage sensitive strains. Repeated transformation of the mutated plasmids to LM0230 confirmed that the observed phenotypes were caused by mutations located on the plasmids. The EOP values and plaque morphology of two enhanced phage resistance mutants were characterized at 30 degrees C and 37 degrees C. These results indicate that this simple procedure can be applied to generate modified plasmids with improved phage resistance, which may be of commercial value.