We investigated the effect of bacterial lipopolysaccharide (LPS) on phospholipid (PL) turnover in human monocytic leukaemia U937 cells. Cells were p re-la belled with [3H]choline, [14C]ethanoIamine and [3H]inositol for 24 h. By monitoring the radiolabel association with cellular PL, the data indicated that LPS (10 μg/ml) drastically altered the catabolism of choline-containing PL; it induced their breakdown by 50% within 20 min. The reutilization of choline or its phosphates for PL synthesis was also suggested as a result of regaining radiolabel in the next 40 min. Choline-containing PL then underwent a second degradation after 60 min; 50% decline in radiolabel was detected at 120 min. In contrast, LPS did not induce the breakdown of phosphatidylethanolamine and phosphatidylinositol through phospholipase C/phospholipase D (PLC/PLD). No significant redistribution of the radiolabel in PL was detected in any cases during chasing. The data clearly indicate that LPS stimulates phosphatidylcholine breakdown, implying that the liberation of phosphatidic acid or diacylglycerol via PLC/PLD reaction may be relevant to the initiation of LPS-induced monocytic activation.