The kanamycin resistance plasmid Rts1 restricts the growth of bacteriophage T2, T4 and T6. The DNA of these phage contains hydroxymethylcytosine (HMC) in place of regular cytosine and is modified by glucosylation. When HMC is not glucosylated, as in the DNA of glucosyl transferase-deficient T4 phage, this restriction becomes less apparent, a phenomenon not observed with any other known restriction systems. On the other hand, glucosylation of HMC in T6 phage leads to a less efficient restriction, while restriction of bacteriophage T2 remains unchanged. The modulating effect of glucose cannot be seen when cells contain a large amount of this enzyme, as in the case when multiple copies of its determinant are present in the cells. T-odd phage and bacteriophage lambda are not restricted by Rts1 suggesting that the restriction is specific to DNA containing HMC. The restriction phenotype is due to a single gene coding for a polypeptide of 293 amino acids. This enzyme has been named PvuRts1I. A gene with the sequence motifs similar to modification enzymes was found upstream of the gene coding for PvuRts1I. This gene, however, neither modifies the restriction phenotype of PvuRts1I, nor codes for detectable modification enzyme. T4 mutants with increased resistance to PvuRts1I appear to have deficiency in their beta-glucosyl transferase enzyme.