<p>The purpose of the present study was to investigate the effect of MatriStem derived from porcine bladder on reparative dentin formation and pulp inflammation in vivo compared with mineral trioxide aggregate and calcium hydroxide that have been used for vital pulp therapy. Exposed pulp tissue in the maxilla first molar of 8-week-old Wistar rats was covered with those materials. The rats were sacrificed 4 weeks after the operation. We performed histological examination of hematoxylin and eosin-stained sections of teeth that underwent direct pulp capping, followed by morphometric analysis of formed reparative dentin and evaluation of pulp inflammation. At 4 weeks, MatriStem formed highly compact, intensively reparative dentin with dentinal tubules. MatriStem induced less pulp inflammation than calcium hydroxide. This study suggests that MatriStem is a biocompatible, biodegradable, and bioactive material with growth factors such as bone morphogenetic proteins for dentin regeneration.</p>