AbstractThe effects of eight organic and eight inorganic compounds on light emission by two cell‐free bioluminescent systems were compared to the effects of these substances on light emission by whole cells ofPhotobacterium phosphoreum(Microtox® strain). Crude luciferase preparations from the firefly (Photoniuspyralis) andVibrio (Photobacterium) fischeriwere exposed to the toxicants at or near the Microtox EC50values for 0.25, 5, or 15 min and then reacted with the necessary reagents for luminescence (ATP for the firefly reaction; NADH, FMN and n‐decylaldehyde for the bacterial extract). The light emission values, relative to controls, were then compared to those obtained after exposure ofP. phosphoreumcells to the toxicants for the same duration. The three systems were found to differ in both response time and sensitivity. The firefly extract was found to be most sensitive to inorganics while whole cells ofP. phosphoreumwere most sensitive to organics after 0.25 min of exposure. However, after 5 and 15 min both systems were nearly equivalent in their sensitivity to the test toxicants. TheV. fishcericell extract was found to be extremely sensitive to three of the inorganics tested at 0.25 minutes, but was either unaffected or stimulated by the remaining inorganic and organic compounds at all times examined. Our results indicate that the sensitivity of detection of certain toxic compounds may be optimized by selection of a particular luminescent sy