cis-.DELTA.2-11-methyl-Dodecenoic acid is a diffusible signal factor (DSF) in extracellular microbial and fungal communication systems. In a DSF bioassay, the minimum concentration of cis-.DELTA.2-11-methyl-dodecenoic acid required for induction of a DSF biosensor was about 0.5 µM, which is 200-fold lower than that of the conformational isomer (trans-.DELTA.2-11-methyl-dodecenoic acid) and 20,000-fold lower than that of the corresponding saturated fatty acid (11-methyl-dodecanoic acid).