The fact that human populations differ in their normal gene frequencies was first shown by the Hirszfelds in 1918 in their survey of the ABO bloodgroups in soldiers and others of different nationalities. The Mendelian basis of the ABO blood groups was already known in 1910, though several years were to pass before the exact mechanism of their inheritance was established by Bernstein in 1924. Included in the Hirszfelds' material were the first samples of tropical populations, e.g. from Cambodia, Vietnam, French West Africa, and the central provinces of India. Over the years, knowledge of gene frequencies in tropical populations increased gradually, but then it received considerable impetus some two decades ago with the development of the International Biological Program. Indeed, much of the present information on human genetic variation in the tropics derives from investigations carried out under its auspices. Information on the genetic constitution of populations was collected with the intention of using it as a variable to be controlled in comparisons of groups inhabiting different environments, and also in its own right with a view to understanding the affinities and differentiation of the communities investigated. At that time, however, although most of the polymorphic blood group systems had been established, the usefulness of electrophoresis as a technique for distinguishing genetic variants was only just beginning to be understood and applied. Hence from the surveys of that period genetic information is restricted to gene frequencies in most polymorphic blood group systems, and a few serum and red cell enzyme and other protein systems.