Hypoxanthine phosphoribosyl transferase (HPRT, E.C. 2.4.2.8) enzyme was purified approximately 200-fold from chick brain tissue by acid precipitation, heat treatment, ammonium sulfate precipitation and DEAE-Sephadex column chromatography. The subunit molecular weight of chick brain HPRT was about 27,000 by SDS-PAGE. The polyclonal antibody against chick brain HPRT was obtained from guinea pig. The recognition by this antiserum of purified chick HPRT was confirmed by the Ouchterlony immunodiffusion test and also enzyme immunoprecipitation assay. The antiserum immunoprecipitated all the chick HPRT enzyme activity, however, failed to immunoprecipitate purified human blood HPRT or HPRT from extracts of cultured human cells. It also failed to immunoprecipitate HPRT enzyme extracted from cells or tissues of mouse, frog, and yeast. The carp HPRT showed slightly cross-reactivity of about 30% with chick brain HPRT. This polyclonal antibody could be used in further investigations of the phylogenetic and epigenetic implications of the distribution of HPRT in the kingdoms of organisms.