“…This hy pothesis had been suggested by two groups of observations: (1) according to some authors [Armstrong, 1938;Bibby and V an Kestern, 1940;Po lice, 1960;Benagiano, Moschella and Sacca, 1962, in contrast with McClure, 1956 andT empestini, 1966] sound enamel from sound teeth contains more fluoride than sound enamel from carious teeth; (2) accord ing to Backer D irks [1963,1967], the effect of fluoridation in reducing dental caries is not the same for different surfaces of the tooth; in the Tiel-Culemborg experiment the caries protective action of fluoride, ex pressed as percentage reduction in caries, was found by Backer D irks to be greater on the approximal, buccal and lingual surfaces than on the oc clusal surfaces, and greater on the incisors than on the posterior teeth. In an earlier investigation Marci and Scavizzi [1967] examined, in a clini cal caries study, the teeth of residents of a low fluoride area (Rieti).…”