A convenient method for the biosynthesis of 12L-hydroxy-5,8,10,14-eicosatetraenoic acid (HETE) from archidonic acid with human platelets was developed, and milligram quantities were made for biological and spectroscopic characterization. This compound has only weak activity in two of five biological prostaglandin assays and no activity in the other three. The 13C and 1H NMR spectra of HETE were obtained and the signal due to each carbon atom was assigned.