It has been proposed that the so-called intact surface zone of carious lesions of enamel could restrict the ingress of mineral ions and hinder remineralisation. The present study was intended to determine the role of organic (proteinaceous) material in restricting the movement of mineral ions into carious lesions in vitro. Natural carious lesion surfaces were divided into two halves. The experimental half was de-proteinised using hypochlorite, the control half remained untreated. The whole tooth was exposed to 45Ca in solution, and 45Ca uptake into experimental and control tissue was measured by image analysis of autoradiographs prepared from lesion sections. The results indicated that uptake was improved by removal of organic material.