Data from a wide range of studies on pulp response to materials in rats and dogs and a series of gnotobiotic experiments designed to investigate pulp response to caries have been reviewed for evidence of migration of materials and bacteria within the pulp. Migration of materials after direct pulp capping was unusual in the rat molar pulp. In the dog, by contrast, migration of materials from direct pulp capping sites was frequently observed. When material particles were contained within phagocytic cells, there was little evidence of any pulp response; free material within the tissues was frequently associated with inflammation or necrosis. In the gnotobiotic studies, phagocytosis of strains of Streptococcus mutans by the pulp in carious teeth was seen.