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It has been recognized for some time that the dental pulp may be invaded by microorganisms either through pulpal exposures or by a vascular route. Robinson and Boling' studied the localization of streptococci and staphylococci in pulps of adult cats, some with naturally acquired infections and others with artificially induced bacteremias. In these over-all findings, pulps of 72 per cent of operated teeth and 8.5 per cent of control teeth contained bacteria when the animals were sacrificed. Robinson and Boling1 were able to demonstrate that the severity of pulpal inflammation increased the chance of bacterial localization, but they were unable to correlate the age of the lesion with the localization of microorganisms. In comparison, Fox2 and Rigdon,3 working with tissues other than dental pulps, were able to show that the localization of bacteria from a hematogenous source occurred more frequently in severely inflamed localized lesions, than in less severel inflamed lesions in a similar location. In addition, younger lesions were more susceptible to localization than older ones.The purpose of this investigation was to study the influence of irritation due to cavity preparation, filling materials, and medicaments on the localization and retention of blood-borne bacteria in pulpal tissues. EXPERIMENTAL METHODSAn albino strain of Wistar rats, between the ages of 75 and 105 days, weighing approximately 150 gm., was selected for experimental study. Earlier reports by Maurice and Schour4'5 and Weider, Schour, and Mohammed6 and preliminary experiments in our laboratory have indicated that the first maxillary molars of rats are suitable for cavity preparations and studies on pulpal reactions.A strain of Staphylococcus aureus was selected as a test organism because it demonstrated moderate virulence for the rat and was not fatal if injected intravenously in relatively large numbers. Furthermore, this organism was easy to isolate and identify by the use of a selective medium.* Experimental animals were anesthetized by intraperitoneal injections of weightgraded doses of sodium nembutalt and were secured in a mouth prop.7 Samples of saliva were streaked on the selective medium and examined later for the presence of S. aureus.