Do tracers trace?

O NLY A FEW YEARS after the discovery of artificial radioisotopes by the Joliot-Curies in 1934, de Hevesy, Chievitz and Ottesen used tracers in hematology. Even today, there are few areas in clinical medicine where as many radioisotopes are used both for investigative and diagnostic purposes as in hematology. Consequently, the methods of tracer studies are often described in the hematologic journals. The methods of interpreting tracer studies are more rarely mentioned, although these methods were described ( for nonradioactive drugs ) as early as in 1937.’ The reason for reviewing them here is the