HaeIII, BsrI and NgoII are isochizomers that recognize the sequence GGCC while EaeI and CfrI recognize the overlapping sequence YGGCCR. It has previously been shown that all these enzymes are inhibited by cytosine C5-methylation within the recognition sequence. The methylation sensitivities of these enzymes to cytosine N4-methylation have not been previously reported. In this paper we present data demonstrating that all these enzymes, except NgoII, are inhibited by cytosine N4-methylation of the second 5' cytosine residue within the recognition sequence.