AbstractThe major finding of this work is that there are specific restriction enzyme inhibitors present in“purified”tumor DNA which cause partial digestion patterns when tumor DNA is digested by standard procedures with any of three commonly employed restriction enzymes (Hind Ill, Kpnl, Xbal). These aberrant patterns are not seen when DNA of cell lines derived from these tumors is digested. Thus, when working with tumor DNA these restriction enzymes should be used with caution.