The less effective killing by milk cells may be caused by the phagocytosed lipid. The powerful humoral bacteriostatic effect of milk, dependent on IgA antibody and lactoferrin, is inactivated by the addition of iron.3 Our data suggest that the bactericidal effect of milk cells is also negligible when iron is added but could contribute significantly in the normal iron-free state, as is indicated by in-vivo experiments.6 These also suggest that the cells survive in the gastrointestinal tract long enough to produce an effect.Epidemiological studies have shown that breast-fed babies are less susceptible to infection than artificially fed babies.' Our results establish a further mechanism for this and strengthen the case against the use of iron-containing complementary feeds.3 There is little prospect of replacing cells in bovine-milk feeds, and our findings raise further problems in planning breast-milk banks.We thank our colleagues, and the mothers at the Mothers' Hospital for the samples. Science, 1973, 56, 733. 15 Ammann, A J, and Stiehm, E R, Proceedings of the Society for Experimental Biology and Medicine, 1966, 122, 1098. 12 Ballow, M, et al, Clinical and Experimental Immunology, 1974, 18, 257. 13 Yamamura, M, and Valdimarsson, H, Scandinavian Journal of Immunology, 1977, 6, 593. (Accepted 28 March 1978) Plasma arginine vasopressin concentrations and antidiuretic action of carbamazepine W P STEPHENS, J Y COE, P H BAYLIS British Medical Journal, 1978, 1, 1445-1447 Summary and conclusions Twelve subjects given thvrapeutic doses of carbamazepine showed no change in their plasma electrolyte concentrations. Ten of the 12 had abnormal water metabolism, however, their ability to excrete water loads being Good Hope General Hospital, Sutton Coldfield, West Midlands W P STEPHENS, MB, MRCP, medical registrar J Y COE, MB, BS, senior house officer in medicine