For the safety extirpation in case of acute pulpitis with painless procedure, lots of pulp devitalizers have been designed for a long time. Authors have manufactured experimental pulp devitalizer with adding some other constituents for main components of paraformaldehyde and local anesthetics. The purpose of this study was to observe the histopathologic tissue response in pulp tissues by applying newly developed safety pulp devitalizer to the human tooth. For this experiments 5 human teeth in 4 patients (ages of 26 to 49) were used. In case of acute pulpitis with pulp exposure, divided into two groups; One group, carious dentin removed, and then was applied newly developed devitalizer directly to exposed small pulp tissues. Other group, was exposed the pulp chamber, pulpotomized and then the devitalizer was applied to remaining pulp tissues. The teeth were extracted on 3rd and 7th day after operation respectively. For the control group, Depulpin? was used as pulp devitalizer. All the extracted teeth were fixed in 10% neutral formalin solution, decalcified in Plank-Richlo solution, embedded in paraffin, sectioned 6-8㎛ in thickness, stained with hematoxylin-eosin stain method, and examined under microscope. Attained results were as follows; 1. There were no difference in histopathologic aspects either exposed or not pulp chamber, the extent of necrosis of pulp tissue and destruction of odontoblast was similar to that on the 3rd day of control group and both the 3rd and 7th day of the experimental group. 2. For the necrosis of pulp tissue and destruction of odontoblast on the 3rd day of control group exposed pulp chamber, was similar to that on the 3rd day of experimental group exposed small pulp tissue. The experimental pulp devitalizer could reduce the side effects, which reduced the amount of paraformaldehyde components, adding other constituents, reduced the duration of pulp devitalization, and useful whether exposed or not the pulp chamber.