Various leukotriene analogues were tested for their capacity to raise the cytosolic free calcium concentration, [Ca2+]i, and to stimulate exocytosis in human neutrophils. Their order of potency for both parameters was LTB4 greater than the stereochemical isomer of LTB4, (5S, 12S)-LTB4 much much greater than the sulphidopeptides LTD4, LTC4. The correlation between [Ca2+]i and secretion indicates that an increase of [Ca2+]i above a threshold level of about 300 nM is necessary for stimulating secretion with LTB4. This threshold is about an order of magnitude higher than that required for the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe). The increase in [Ca2+]i elicited by LTB4 was unaffected by increasing cellular cAMP, while secretion was completely inhibited. These results indicate, that similar to fMet-Leu-Phe, leukotrienes generate other signals in addition to [Ca2+]i elevations. Contrary to previous claims, leukotrienes stimulate polyphosphoinositide hydrolysis, as indicated by the increase in [3H]inositol trisphosphate, InsP3, observed upon stimulation of myo[3H]inositol-labelled neutrophils with LTB4 or (5S, 12S)-LTB4. The two InsP3 isomers [Ins(1,4,5)P3 and Ins(1,3,4P3] were separated by high-pressure liquid chromatographed and, as reported for other cell types, the formation of Ins(1,4,5)P3 precedes that of Ins(1,3,4)P3. Maximal stimulatory doses of LTB4 or (5S, 12S)-LTB4 produce about 50% the amount of InsP3 generated by equimolar concentrations of fMet-Leu-Phe. The present observations suggest that, though the transmembrane signalling systems activated by LTB4 and fMet-Leu-Phe are the same, the different efficacy of these two agonists at stimulating neutrophil functions is due, at least in part, to a different degree of activation of phospholipase C.