Alkaline phosphatase of rat dental pulp was separated into two forms by anion-exchange chromatography, gel-filtration and electrophoresis. The minor activity (AP I) was eluted at 0.1 M of NaCl from a DEAE-cellulose column, and the major activity (AP II) in the broad range of approx. 0.15-0.5 M. These two enzyme activities gave rise to distinct peaks on a TSK-GEL Toyopearl (Fractogel TSK) HW 55-S column and showed different electrophoretic behaviours on both sodium dodecyl sulphate (SDS) and non-SDS polyacrylamide gels. The molecular weights of AP I and AP II were estimated by gel-filtration to be 130,000-150,000 and above 500,000, respectively. On SDS gel electrophoresis, the molecular weight of AP II was changed to 130,000, while AP I had a molecular weight of 150,000. Judging from the sensitivities to heat and some inhibitors, AP I and AP II are biochemically indistinguishable from each other.