Non-esterified fatty acid (NEFA) contents were determined (1) in cow, goat, mare and human plasmas or sera and (2) in bovine serum albumin solutions by thin-layer and gas-liquid chromatographic methods which were compared with 6 chemical methods of determination and 2 enzymatic methods (manual and automatic). The chemical methods combined two extraction techniques (isopropanol-heptane and silicic acid-diisopropyl ether) and three determination methods (titrimetry and colorimetry using either phenol red or copper soaps). Correlations between the nine various methods were high, but in some cases regression lines were different from the bissector and species-dependent. The putative occurrence of interfering compounds or the possible effect of between-species differences in NEFA composition have been discussed. The choice of a fast method depends primarily on practical considerations, but the need to standardize each method for a given species has been stressed. Automatic enzymatic assay seemed to be suitable for routine analyses.