The SbcCD protein is a member of a group of nucleases found in bacteriophage T4 and T5, eubacteria, archaebacteria, yeast, Drosophila, mouse and man. Evidence from electron microscopy has revealed a distinctive structure consisting of two globular domains linked by a long region of coiled coil, similar to that predicted for the members of the SMC family. That a nuclease should have such an unusual structure suggests that its mode of action may be complex. Here we show that the protein degrades duplex DNA in a 3'-->5' direction. This degradation releases products half the length of the original duplex suggesting simultaneous degradation from two duplex ends. This may provide a link to the unusual structure of the protein since our data are consistent with recognition and cleavage of DNA ends followed by 3'-->5' nicking by two nucleolytic centres within a single nuclease molecule that releases a half length limit product. We also show that cleavage is not simply at the point of a single-strand/double-stand transition and that despite the dominant 3'-->5' polarity of degradation, a 5' single-strand can be cleaved when attached to duplex DNA. The implications of this mechanism for the processing of hairpins formed during DNA replication are discussed.