ONE of the most pressing challenges in the design of molecular (1-7) is to achieve visual discrimination between enantiomers. A simple monitoring system for distinguishing between the left- and right-handed forms of chiral drugs would be extremely useful in pharmacology(8). A molecular sensor that achieves this would ha, e to translate an enantioselective molecular recognition event into a discernible colour change. Here we describe a system of this sort, in which a proton transfer and conformational change in a chiral calixarene-based receptor brought about by recognition of a chiral substrate cause spectral shifts in two chromophores attached to the binding cavity of the calixarene, Because the spectral shift in one chromophore is greater for one enantiomer of the substrate than for the other, binding is accompanied by a colour change that can be observed visually.