The enzymes for luminescence in Vibrio fischeri are induced only when a sufficient concentration of a metabolic product (autoinducer) specifically produced by this species accumulates. It has previously been shown that the autoinducer is 3-oxohexanoyl homoserine lactone and that it enters the cells by simple diffusion. To further study the mechanism of induction, we have synthesized several analogs of the autoinducer. The analogs were tested with V. fischeri for their inducing activity and for their ability to inhibit the action of the natural autoinducer. The compounds were found to display various combinations of inducing and inhibiting abilities. None of the compounds tested appeared to have any effect on cells of V. harveyi strain MAV or Photobacterium leiognathi strain 721, but several of the compounds decreased light output by P. phosphoreum strain 8265. These studies show that 1) the site of action of the autoinducer is not highly sterically constrained 2) the autoinducers of other species of luminous bacteria are likely to be quite different from that of V. fischeri and 3) a simple mode in which one autoinducer molecule binds to a single receptor protein site and thus initiates luciferase synthesis is inadequate. The analogs should prove useful in the study of the binding site and mode of action of the autoinducer.