Recent positive drug tests by two cyclists suggest there is a new, cutting-edge substance making its way to athletes looking for performance-enhancement: FG-4592, an experimental drug that increases production of red blood cells but has not yet been approved for human consumption.

Athletes have long found surreptitious means of obtaining banned performance-enhancing drugs, but with FG-4592, there actually may be a far more straightforward way — simply by ordering it from chemical-supply companies online.

And that, said Don Catlin, an expert on testing for banned substances, is something new to him. Athletes have gotten banned drugs from websites in China, he said, but what they receive is not always what they tried to order. The companies selling FG-4592 are not marketing it to athletes or individuals who want to take it; they emphasize that what they are selling is a chemical that is intended only for researchers.

But according to Catlin, a consultant on drug testing and former director of the U.C.L.A. lab that performs drug tests for the Olympics, that may not stop a determined athlete.