Mucolipidosis II (ML II, I-cell disease) and mucolipidosis III (ML III, pseudo-Hurler polydystrophy) are human autosomal recessive genetic disorders resulting from deficient UDP-N-acetylglucosamine:lysosomal enzyme precursor N-acetylglucosamine-1-phosphotransferase (GNPT) activity. Normally, this enzyme is involved in the processing of most lysosomal enzymes. Cultured fibroblasts from individuals with either disorder are deficient in a broad array of lysosomal enzymes as a result of the diminished GNPT activity. We report the correction of this phenotype by fusing transformed ML III cells generated for this study to lethally irradiated rodent cells. This method of gene transfer does not require selection for the gene of interest, animal models, nor any knowledge of the gene product except a screening method for its presence. It has generated corrected cell hybrids that contain approximately 1% hamster-derived sequences. These cell lines, which contain the hamster analogue to the human phosphotransferase gene, are useful for the molecular cloning of the gene defective in ML II and ML III.