AbstractThe Ames Salmonella mutagenicity assay has been used to assess the metabolic activation of the following polycyclic aromatic hydrocarbons by human colonic microsomes (S9) and a cell‐free extract ofBacteroides fragilis(Bf): 2‐aminoanthracene, 1‐naphthylamine, 2‐naphthylamine, 2‐aminofluorene, 2‐acetylaminofluorene, anthracene, benzo(a)pyrene, 3‐methylcholanthrene, 7, 12‐dimethylbenzanthracene, acridine, 9‐aminoacridine and 3‐methylindole.2‐Aminoanthracene and 2‐aminofluorene were the only compounds activated. In both cases, activation was dose‐dependent and 2‐aminofluorene exhibited synergistic activation by S9+Bf, as has previously been demonstrated with 2‐aminoanthracene.The organospecific aliphatic colonic carcinogen, 1, 2‐dimethylhydrazine was not activated in this system. Metabolic activation by S9+Bf is thus restricted to aromatic compounds with three rings and an amino group in position 2.These findings are consistent with enzymic substrate specificity, and are compatible with an enzymic basis for activation of polycyclic aromatic hydrocarbons byB. fragilis, present in large concentrations in the coloni