A Chinese hamster cell mutant has been described with little or no activity of succinate dehydrogenase (Soderberg et al., 1977). We describe here the selection and characterization of human-hamster hybrids obtained from the fusion of these mutant cells and human lymphoblasts or HT1080 fibrosarcoma cells. The presence of human chromosome 1, identified by cytogenetic techniques and isozyme analysis, is correlated with the restoration of succinate dehydrogenase activity in the hybrids, and segregants are described in which the loss of all or part of human chromosome 1 has also led to a loss of this activity. We present results which support the claim that the mutation in the Chinese hamster cells is in one of the two structural genes for the 70,000 and 30,000 dalton peptides, respectively, which constitute succinate dehydrogenase. One of these two genes is therefore mapped on human chromosome 1.
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