It is known that both platelet-activating factor (PAF) acetylhydrolase and phospholipase A2can inactivate PAF. An enzyme with the ability to convert labelled PAF into lyso-PAF was detected in the culture medium of mouse calvaria cultured in serum-free BG Jb medium containing 0.1% bovine serum albumin. The enzyme was unable to degrade labelled long chain diacylphosphatidylcholine, and was inert to addition of Ca2+(10 mM) or EDTA (10 mM). The enzyme was inhibited by diisopropylfluorophosphate in a dose-dependent manner. Moreover, the enzyme was found to be acid-labile, in a similar manner to PAF acetylhydrolase in mouse plasma. Based on these data, the enzyme was identified as PAF acetylhydrolase.It is known that PAF acetylhydrolase in rat plasma is different from that in rat kidney cortex in its sensitivity to proteases. PAF acetylhydrolase in mouse calvaria was insensitive to trypsin and pronase, like PAF acetylhydrolase in mouse plasma, while PAF acetylhydrolase in mouse kidney cortex was sensitive. PAF acetylhydrolase activity in the culture medium increased time-dependently for at least 48 h. Treatment of calvaria for 12 h with cycloheximide (1μg/ml), an inhibitor of protein synthesis, almost completely prevented the appearance of acetylhydrolase activity in the culture medium. Thus, the results suggest that mouse calvaria synthesize PAF acetylhydrolase and release it extracellularly under no stimuli, and that the properties of the acetylhydrolase in calvaria are similar to those of the enzyme found in plasma. Therefore, mouse calvaria may be one of the origins of PAF acetylhydrolase in plasma.