Ouabain resistant mutants of hypoxanthine phosphoribosyl transferase deficient HeLa cells and euploid human fibroblasts were isolated and extensively characterized. These double mutants were used to test the prediction that they would be "universal hybridizers" for intraspecific human cell hybrid production. Hybrids were selected from fusions with human cells of diverse somatic origin. In addition it was shown that multiparental hybrids can be generated using a modification of the selection system, a result which will be useful for gene dosage experiments involving human cells.