CONJUGAL opines secreted by crown gall tumours induce strains of Agrobacterium tumefaciens that are donors of Ti plasmids to produce a diffusible conjugation factor¹. This enhances the conjugal transfer efficiency of the Ti plasmid in other strains of A. tumefaciens. This factor behaves as a secondary messenger, transmitting the environmental information to tra genes. Here we report the use of spectrometry to show that this factor is identical to synthetic N-(β-oxo-octan-l-oyl)-L-homoserine lactone and confirm that the synthetic compound is biologically active. N-(Hexan-l-oyl)-L-homoserine lactone has also been detected. A closely related molecule, N-(β-oxo-hexan-l-oyl)-L-homoserine lactone, autoinduces bioluminescence in the distantly related bacterium, Vibriofischeri². N-Acyl-homoserine lactones thus seem to be conserved molecules in which the length and nature of the lipophilic acyl chain determines the biological function to be regulated. Mutants that do not produce the factor fail to conjugate unless supplied with it in the induction medium (our unpublished data). These data indicate that the conjugation factor is an autoinducer and a key signal molecule in the conjugation system of A. tumefaciens. It is, to our knowledge, the first example of a second messenger molecule in a bacterial conjugation system.