A fast electrophoretic variant of hypoxanthine phosphoribosyltransferase (HPRT) has been detected in Mus musculus bactrianus, a mouse subspecies from Middle Asia (USSR). The electrophoretic HPRT pattern yielded by hybrids between the somatic cell of LMTK- (deficient in thymidine kinase) and the splenocytes of a male of M. m. bactrianus was five-banded. The pattern obtained from the germ cells of the ovaries from 14.5-day-old embryos from laboratory CBA mice X M. m. bactrianus crosses was also composed of five bands. The hybrids between the somatic cells of human fibroblasts X LMTK- cells gave a three-banded electrophoretic HPRT pattern because the asymmetrical heteropolymeric isozymes are probably unstable. Taken together, all the evidence is in favor of a tetrameric structure of mammalian HPRT.