Summary.The major bacterial species in ferret plaque were indetified and the cytontoxicity of selected orginsismsin vitroassessed to determine their porential role in pulpal inflammation. Of the five species investigated in detail the cell fractions ofEscherichia coliandStreptococcus sanguisfrom anaerobic cultures. The release of a lysosomal enzyme was elicited to differing degree by all five bacterial species. While the amount of enzyme released from macrophages was greater, fibroblasts were generally more sensitive, with the media fraction in addition to the cellular fraction affecting cell numbers and enzyme release. The action ofS. faecalison both cell types was shown to be dose‐department. Since the number of bacteria involvedin vivois small these results suggest they would have little direct toxic effect indicating that some other mechanism must be invove