Several species of Actinomyces viscosus were examined using the scanning electron microscope and considerable morphological heterogeneity was seen and one species, Actinomyces viscosus WVU 398B, was observed to undergo a form of differentiation, for which the term 'vacuolation' has been introduced. This form of differentiation can be regulated by inhibitors of peptidyl transferase or translocase. Further examples of the regulation of differentiation were provided by an examination of the interaction of antibiotics and antimetabolites with a species of Bacillus cereus isolated from laboratory dust. 3-chloropropane-1-2-diol exhibited an ability to impair cell division in Staphylococcus aureus and Escherichia coli and this action may be due to the glycerol antagonism of 3-chloropropane-1,2-diol.