In an outermost layer of porcine secretory enamel, metalloproteinases were detected by enzymography with gelatin used as a substrate. When the sample extracted from the outermost layer of the secretory enamel was incubated with calcium ions at 37 degrees C prior to electrophoresis, an increase of the 34-kDa proteinase activity and a decrease of the 76- and/or 78-kDa proteinase activities were observed. The results suggest that the metalloproteinases mediate the conversion from 76- and/or 78-kDa proteinases to the 34-kDa proteinase or the activation of a latent type of the 34-kDa proteinase, and that their activities are regulated by free Ca ions.