Phospholipase A2 activity was measured in human endometrium throughout the menstrual cycle using an assay based on the liberation of oleic acid from 1-palmitoyl-2-[14C]oleoyl phosphatidylcholine. The enzyme was shown to be calcium dependent, to have an optimum pH of 8-9 and an apparent Michaelis constant of 110 mumol/l. Enzyme activity was low in early proliferative-phase tissue (6.08 +/- 1.42 (S.E.M.) pmol oleic acid released/mg protein per min) but rose significantly (P less than 0.01) during the late proliferative phase (10.86 +/- 2.79 pmol/mg per min). There was a tenfold increase in activity 2-4 days after ovulation (45.6 +/- 13.6 pmol/mg per min) which thereafter declined to reach values which at menstruation were not significantly different from those of the proliferative phase (4.5 +/- 1.76 pmol/mg per min). The results indicate that phospholipase A2 activity in human endometrium is related to the stage of the menstrual cycle and suggest that arachidonic acid release may be influenced by oestradiol and progesterone.