Administration of colchicine, inhibitor of microtubules, causes abnormal amelogenesis in the experimental animals. The purpose of this study is to characterize defective rat enamel which is formed after a single administration of colchicine, and to examine the effect of Nd-YAG laser on acid resistance of the defective enamel.<BR>Rats were sacrificed on 12th day after a single subcutaneous injection of colchicine at a dose of 1.5mg/Kg of body weight. Defective enamel was found on the incisors as white bands with rough surface. It was incompletely mineralized.<BR>The defective enamel was exposed to the artificial caries systems before or after irradiation of Nd-YAG laser with continuous wave. The defective enamel had less resistance to the acid demineralization than normal enamel. After irradiation of laser, however, the defective enamel got the resistance at the same level as that of normal enamel.<BR>The results suggest that Nd-YAG laser irradiation on human teeth with hypomineralized enamel may be effective for the prevention of dental caries.