The lipocalin apolipoprotein D (ApoD) is associated in human plasma with lecithin-cholesterol acyl transferase. It has also been found in high concentration in the fluid of gross cystic disease of the mammary gland. Using protein fluorescence quenching, it is shown that ApoD binds arachidonic acid (Ka of 1.6 x 10(8) M-1) and as previously known progesterone (Ka of 2.5 x 10(6) M-1), but neither cholesterol nor any of the other prostanoid molecules examined had measurable affinity. This specific binding of arachidonate, also observable directly, suggests a role for ApoD in the mobilisation of arachidonic acid, and hence prostaglandin synthesis.