The seminal plasma of seven eutherian species, including man, was examined for the presence of free N-acetylaminosugar. Only man had appreciable levels of N-acetylaminosugar in the semen (37-1 mg/100 ml), but in all cases this free N-acetylaminosugar was probably the result of breakdown of polysaccharides by semen glycosidases. High levels of free N-acetylaminosugar thus appear to be peculiar to marsupials.