Summary.A series of deep cavities with and without traumatic pulpal exposures were prepared in the maxillary molars of albino rats. They were left untreasted for various time intervals before killing.Demineralized sections were prepared in a mesiodistral plane and stained to demonstrate the presence that were grossly contaminated, bacteria were detected in the majority of sections. In the roor canaks and periapical tissues, bacteria were detected in only a minority of sections.It is considered that serial sections stained to demonstrate bacteria are necessary for the detection of bacteria that the present in only small numbers in the tissue. The demonstration of small numbers of micro‐organisms in specific zones of the pulp remains problematic. Failure to detect stained bacteria in histological sections is not absolute proof of their absenc