We investigated whether leukotriene B4 (LTB4), a granulocyte inflammatory mediator, is detectable in cerebrospinal fluid using a high performance liquid chromatographic method. In non-pleocytotic cerebrospinal fluid (n = 5) and in cerebrospinal fluid from children with aseptic meningitis (n = 8), LTB4 concentrations were below the detection limit (< 0.2 ng ml-1). In the range 0-20 ng ml-1, the recovery rate of LTB4 that had been added to non-pleocytotic cerebrospinal fluid was > 90%. In cerebrospinal fluid with a polymorphonuclear leucocyte (PMN) count higher than 1,000/ml, LTB4 was detectable in six out of seven specimens with a concentration of 0.35-3.3 ng ml-1. LTB4 concentration was significantly correlated with PMN number. These results, together with observations in animal models, are discussed with regard to a pathophysiological role of LTB4 in bacterial meningitis.