Genetic relations among living species can be deduced from biochemical as well as morphological similarities, but our understanding of fossil species has depended entirely on their morphology. Residual proteins in fossils might provide genetic information, but their small quantity and chemical alterations due to time and environmental agents have prevented the obtaining of species-specific analysis. This report describes a radioimmunoassay capable of detecting extremely small amounts of fossil proteins, such as collagen and albumin. Species-specific proteins have been identified in a frozen Siberian mammoth, a Pleistocene bison, and a series of human fossils that includes Neanderthal, Homo erectus and Australopithecus robustus. This technique promises to provide molecular data on the genetic affinities of fossil and living species.