A substrate probe for intracellular phospholipase A was used to examine the activity of this class of enzyme in rabbit alveolar macrophages. The calcium ionophore A23187, insoluble immune complexes and á-glucan were all capable of stimulating cellular phospholipase A in terms of an increase in probe fluorescence detected by flow cytometry. In contrast, there was no evidence of phospholipase A stimulation in cells challenged with cotton tannin. The results support our previous findings that tannin appears to increase the level of free arachidonic acid in alveolar macrophages via an inhibition of the fatty acid reacylation process rather than the stimulation of phospholipase A(2).