Karyopycnotic and keratinisation indices were determined in buccal smears which had been taken from newborns. An assessment was made of 239 smears which had been obtained from newborns in their first eight weeks of age, with the probands being subdivided into two groups, depending on breast feeding and feeding on powdered milk (Ki-Na).--Both indices were extremely high on the first day after birth, but they declined up to the eighth or 14th days of age which was attributed to postnatal reduction of high foetal oestrogen levels. Both indices then went up, with some difference being recordable between the two groups, depending on the modes of feeding. The keratinisation index of the breast-feeding group was much higher than that of the Ki-Na group, on account of very high oestrogen levels in breast milk, between the fourth and seventh days from delivery, with consequences on mucous membrane proliferation. The keratinisation index declined gradually along with oestrogen drop of breast milk and turned to be equal to that of the Ki-Na group, eight weeks after delivery.--Proliferation of buccal mucous membrane and its importance to efficient defence against infection during the first four weeks of age are discussed.