IN a stimulating article in Nature Pullman and Pullman1 direct attention to the fact that many essential biomolecules contain resonance systems with delocalized π-electrons. The authors are right in stressing their importance for the function of these molecules in life processes; but the authors' claim that this is 'one essential, although apparently hitherto unnoticed aspect of biochemistry' cannot be accepted. I am convinced that many biochemists are already quite familiar with it.