Many analytical methods for eicosanoids require extraction of these substances from biological fluids or media prior to analysis, and even when not essential, extraction can be used to increase both the specificity and sensitivity of assays. Extraction and chromatography of eicosanoids and other solutes depends on their partitioning between two phases that differ in polarity. Although all eicosanoids contain the negatively charged carboxylate anion, they are relatively hydrophobic molecules as a result of the presence of the 20 carbon chain. Exceptions to this are the cysteinyl-leukotrienes (cys-LTs), which have positively charged amino groups. Thus the partitioning of leukotrienes C₄, D₄, and E₄ and their metabolites between these two phases differs from that of other eicosanoids. This is reflected by different behavior of cys-LTs compared to other eicosanoids when they are subjected to extraction and chromatographic analysis (1). It should be noted, however, that N-acetyl-LTE₄, a major metabolite of cys-LTs in the rat, does not contain a free amino group, and thus behaves more like noncysteine-containing eicosanoids.