A specific enzymatic method for the routine measurement of L-leucine in blood samples is presented. The method uses a commercial preparation of tRNAs and amino acetyl-tRNA synthetases for the specific loading of L-leucine into the tRNA(Leu) present, competing with carrier-free L-[U-14C]leucine. The radioimmunoassay-like plot of radioactivity found in the acid-insoluble (tRNA) fraction was used to determine the amount of unlabelled L-leucine of the samples when compared against a standard curve. The interference of L-isoleucine, L-valine and L-alanine was very low.