The binding of some selected food dyes to hydroxyapatite with and without a coating of chlorhexidine was studied. The apatite had bound 0.6 mumol chlorhexidine per g dry weight. The bound chlorhexidine was in equilibrium with a free concentration of 115 microgram ml (128 micron). The dyes investigated were brilliant blue (FD&C Blue No. 1), indigo carmine (FD&C Blue No. 2), tartrazine (FD&C Yellow No. 5), sunset yellow (FD&C Yellow No. 6), amaranth (FD&C Red No. 2), and riboflavin. Riboflavin did not bind to either chlorhexidine-treated apatite or untreated apatite, whereas the other dyes showed a considerable affinity for chlorhexidine-treated apatite as compared with untreated apatite. Because the dyes with binding ability possess two or three acidic groups, and because riboflavin has none, it was suggested that the binding of the dyes is mediated by an interaction between the anionic groups of the dye molecules and the cationic groups of the chlorhexidine molecules. The results are discussed and related to the formation of the brownish discolorations seen on the teeth of patients using chlorhexidine mouthrinses. A mechanism to explain the development of tooth stains is proposed.