Lipocalin type prostaglandin-D-synthase (L-PGDS), also called beta-trace, is an extracellular protein very abundant in compartments beyond blood-tissue barriers, such as the cerebrospinal fluid, the aqueous humor, the amniotic fluid and the seminal fluid. In the latter fluid the major function of L-PGDS does not seem to be the synthesis of prostaglandin D(2) (PGD(2)) from its precursor PGH(2), which is very unstable in aqueous solution. Instead, seminal L-PGDS, an important carrier of bile pigments, retinoids, thyroid hormones and essential fatty acids, would contribute to providing, beyond the blood-testis barrier, thyroid hormones and retinoids to the developing germ cells in the seminiferous tubules and the maturing spermatozoa in the epididymis.