A rapid and precise microfluorometric method for the determination of free fatty acid concentrations in 2-5 microliters of plasma is described. The assay is performed directly on plasma, eliminating the need for extraction with organic solvents, and is based on the quantitation of AMP generated from the formation of acyl coenzyme A in the presence of ATP and acyl CoA synthetase. Because of the sensitivity of this assay, reagent quantities, and thus costs, are significantly reduced compared with previously described enzymatic spectrophotometric methods. In addition, the inclusion of fatty acid-free human serum albumin in the standards corrects the previously reported underestimate of plasma free fatty acids with enzymic methods.