The sweet smell of serum.

You smell. Don't worry, we all do. In fact, studies indicate that every animal has a distinctive smell, a so-called odortype. Moreover, research suggests that rodents, and even people, prefer a mate with a different smell, perhaps as a way of ensuring genetic variability within a population. A study now reports that these identifying odors circulate in the blood, although they're apparently bound to proteins that normally mask the smell.

Much of the research on odortypes has focused on the ability of mice to identify the smell of urine from different mice. Since urine is a complex liquid, scientists haven't had much success identifying the specific odor molecules recognized by mice. Suspecting that serum, the fluid portion of blood, might also carry the odorants, investigators tested whether mice can discriminate among blood samples. They couldn't.

The odor molecules, however, may be bound to other proteins circulating in serum and are freed only when processed by the kidney into urine, says Kunio Yamazaki of the Monell Chemical Senses Center in Philadelphia. In the Feb. 16 Proceedings of the National Academy of Sciences, he and his colleagues show that mice can indeed discriminate among serum samples-nearly as well as among urine samples--if the liquid is first treated with a protease. This enzyme, which breaks down proteins, apparently frees the odorants, says Yamazaki.