Forty six synovial fluid samples from 42 patients with inflammatory joint disease were analysed by reversed phase high performance liquid chromatography to determine 5-lipoxygenase products, specifically dihydroxyeicosatetraenoic acids (diHETEs). Twenty eight per cent of the fluids which were assayed had one or more products of 5-lipoxygenase activation. Seven fluids contained leukotriene B4 (0.1-28.1 ng/ml); three fluids had low concentrations of 20 carboxy/hydroxy-leukotriene B4 (0.01-0.05 ng/ml); three samples had leukotriene B4 isomers (1.5-2.4 ng/ml); and four fluids contained 5,15-diHETE (2.3-16.4 ng/ml). There was a poor correlation between synovial fluid white blood cell counts and evidence of 5-lipoxygenase activation. Several fluids contained unidentified compounds with spectra similar in shape to that of trienes, but the lambda max values of these unidentified compounds were different from those of known leukotrienes. A septic peritoneal exudate and a septic pleural fluid had concentrations of leukotriene B4 and leukotriene B4 isomers and metabolites in a range similar to those found in synovial fluids.