Casein (bovine milk phosphoprotein) at 2 per cent (w/v) in drinking water reduced the extent of fissure and smooth-surface caries of male Sprague-Dawley rats consuming a solid cariogenic diet. Whey protein (the non-phosphorylated protein group of bovine milk) also at 2 per cent (w/v) in the drinking water produced a smaller reduction and only of fissure caries. There was no significant difference in salivary-gland function (as determined by protein concentration), or in the amount or frequency of cariogenic diet consumed. The finding that a 2 per cent solution of whey protein reduced the extent of fissure caries in animals consuming a solid diet containing 26 per cent whey protein suggests that the anticariogenic action is mediated by the protein being in solution. These results suggest a topical anticariogenic action for dietary protein.