DESPITE numerous findings that intact enamel is permeable in certain circumstances to various dyes, radioactive tracers and other chemically identifiable substances1, the ionic-sieve behaviour of enamel demonstrated as long ago as 1929 by Klein and Amberson2 has been largely ignored. Their work could have an important bearing on the whole problem of ionic transport through enamel in vivo, and its chemical stability, so a more systematic study of membrane phenomena associated with enamel has been undertaken. The selectivity to ionic transport of enamel caps of sound human teeth, enamel sections and membranes prepared from a synthetic hydroxyapatite powrder in a poly (methyl methacrylate) matrix has been investigated by e.m.f. measurements of concentration cells using KCl, NaCl and CaCl2 solutions at pH. 7.4 and 25° ± 1° C. The technique for mounting the teeth and measuring the cell e.m.f. has been described in the literature3. The concentration of the electrolyte was maintained at 0.100 M on one side of the cell and on the other side varied in steps from 0.100 M to 0.002 M. All cell e.m.f.s were measured under equilibrium conditions as assessed by the constancy of the e.m.f. over a period of days. The establishment of equilibrium conditions on changing from one salt solution to another was protracted; following the replacement of CaCl2 solutions by KCl solutions, for example, the average time to re-establish equilibrium was of the order of 8 weeks.