Human cerebrospinal fluid (CSF) is the richest source of prostaglandin (PG) D synthase, a key enzyme in sleep regulation, having a specific activity of 20-130 nmol/min/mg protein that is almost two orders of magnitude higher than that of crude rat brain homogenate (2-7 nmol/min/mg protein). PGD synthase was purified from human CSF approximately 8-fold to apparent homogeneity in a yield of 6% by ammonium sulfate fractionation followed by column chromatographies on phenyl-Sepharose and DEAE-cellulose. The purified enzyme displayed enzymatic properties almost identical to those of rat brain PGD synthase in terms of Mr, specific activity, optimum pH, Km value, and SH requirement. The purified PGD synthase cross-reacted with an antibody against rat brain PGD synthase and also with that against human beta-trace, a major protein in the CSF. Furthermore, beta-trace cross-reacted with rat brain PGD synthase antibody. The N-terminal sequences of human PGD synthase, beta-trace, and purified PGD synthase from human CSF were essentially identical. These results clearly show that beta-trace is structurally and enzymologically identical to PGD synthase.