The concept of intra-oral cariogenicity tests, using naturally accumulated plaque, was pioneered by Koulourides (Koulourides et al., 1974). Tests rely on changes in microhardness of dental enamel after exposure to substrates. We used this model, with significant modifications, to determine the possible caries-protective effect of fluoridated milk. Custom-made cast chrome intra-oral appliances were made to fit the lower arches of volunteers. Four removable, highly polished 3 x 4 mm gauze-covered bovine enamel blocks were slotted into the appliances. These were worn for 48 h so that plaque would build up. The enamel was color-coded with composite (Kerr Kolors, Kerr, Romulus, MI) to ensure error-free removal and immersion extra-orally in the coded test substrates.