Prostaglandin D₂ synthase (PGDS) is a major epididymal secretory protein in several species. We quantified PGDS in ram and bull semen using a specific antiserum. Strong variations in PGDS concentration existed between animals. In the bull, the highest concentrations were found preferentially in animals with normal or high fertility, as was previously suggested. However, low concentrations were found in males with all ranges of fertility, suggesting that the function of PGDS either is not necessary for male fertility or can be assumed by other proteins when its concentration is low. In the ram and stallion, cDNA and deduced protein sequences of PGDS were obtained by reverse transcription-polymerase chain reaction and showed that PGDS possessed the sequences involved in the three-dimensional folding characteristic of the lipocalin family and a cysteine at position 65 that is involved in the enzymatic activity. The enzymatic activity of PGDS was estimated in the ram by in vitro incubation of epididymal-isolated tubules with radioactive arachidonic acid. Prostaglandin (PG) D₂ represented approximately 10% of the PGs produced in the lumen, irrespective of the presence or absence of luminal PGDS, suggesting that this protein is not involved in PGD₂ biosynthesis. These results were corroborated by the absence of conversion of PGH₂ to PGD₂ when epididymal fluids were incubated with PGH₂. In the rat, inhibition of PG biosynthesis in vivo by nonsteroidal anti-inflammatory drugs for 60 days did not change spermatozoa mobility or male fertility. It is likely that PGDS, which has a structure similar to that of lipocalin, functions as a lipophilic carrier protein, because we have shown that epididymal PGDS binds retinoic acid and testosterone in vitro.