Small samples of skeletal remains of three humans and three nonhuman animals were collected. Although the species were known prior to bone analysis, the samples were too small and fragmentary to reveal morphological markers of the species. This article describes in detail the procedure of protein radioimmunoassay (pRIA) applied to the samples by a researcher who had no knowledge of the source of the samples. The technique consisted of protein extraction followed by a solid-phase double-antibody radioimmunoassay, using controls of antisera (raised in rabbits) and radioactive (iodine-125) antibody of rabbit gamma globulin (produced in donkeys). Species determination resulted from an evaluation of radioactivity uptake. The test distinguished human from nonhuman species, even in old bone, although age may lead to low amino acid levels and protein preservation problems. Because such small quantities are required, pRIA can be used to test for species prior to DNA analysis. 2 tables, 3 figures, and 18 references