A polyenoic fatty-acid isomerase (PFI) from a red marine alga was used to convert anandamide (5Z,8Z,11Z,14Z-eicosatetraenoyl-N-ethan olamide) to the 5Z,7E,9E,14Z-eicosatetraenoyl-N-ethanol amide isomer. This novel eicosanoid, termed conjugated triene anandamide (CTA), was assessed for its ability to bind to the cannabinoid receptor in rat brain membrane preparations. CTA is a high affinity cannabimimetic substance whose novel structure provides new insight into structure-activity relationships of cannabinoid receptor ligands. These experiments illustrate the utility of enzymes isolated from marine organisms in the development of pharmacological probes.