We have detected a novel splice variant of the hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene in two human oocytes and four preimplantation embryos from the 4-cell to the 8-cell stage of development. The novel HPRT transcript lacks exons 4, 5 and 6 of the normal HPRT gene. The same parental origin for the two oocytes and two of the preimplantation embryos, in which the alternatively spliced transcript was detected, might suggest that the alternative splicing is influenced by genetic background. Mutations in the HPRT gene which cause alternative mRNA splicing are implicated in Lesch-Nyhan syndrome. However, the relatively high frequency of detection of this novel HPRT transcript described here (6/109 oocytes and preimplantation embryos) suggests that it is not involved in Lesch-Nyhan syndrome. It is probable that the alternative HPRT transcript is derived from the aberrant splicing of a small percentage of the total mRNA produced from normal HPRT alleles. The presence of this alternative transcript in human preimplantation embryos may complicate an reverse transcription-polymerase chain reaction-based preimplantation diagnosis of Lesch-Nyhan syndrome.