In contrast to the phorbol ester oxidative response, which only develops during dimethylsulphoxide (DMSO)-induced differentiation of the human leukemic myeloblast HL-60 cell-line, the endotoxin response was observed in undifferentiated and differentiated cells. The Ca2+ response to endotoxin, detected in both differentiated and undifferentiated HL-60 cells, consisted of a transient 10-50 nM increase in intracellular Ca2+. A very slow, irreversible increase in intracellular Ca2+ was detected at high 1-100 micrograms/ml endotoxin concentrations, and this effect, and the inositol phosphate response, correlated with the surfactant activities of various endotoxins and Lipid A. Arachidonic acid and sodium arachidonate 1-50 microM stimulated a large 200-500 nM and transient Ca2+ response in undifferentiated HL-60 cells, which was significantly greater than that elicited by 1-50 microM eicosapentaenoic acid, and was not observed at similar concentrations of arachidonic acid methyl ester or myristic acid. These concentrations (1-50 microM) of arachidonic acid were observed to have surfactant activities on the plasma membrane. At lower arachidonic acid concentrations a marked potentiation of both Ca2+ and oxidative responses to the chemotactic peptide fMet-Leu-Phe was detected. It is possible that the arachidonic acid released during phospholipase A2 activation of neutrophils may be involved in cellular cross-talk and, at higher concentrations, in directly activating Ca2+ and superoxide production. It is also possible that previously reported effects of endotoxin at high concentrations are an in vitro artefact of surfactant properties of endotoxin.