Karyotypes of hybrid cells were studied in continuous uncloned cultures by Q- and C-bandings. Cultures were initiated by virus-mediated or spontaneous cell fusions from normal human diploid fibroblasts and mouse heteroploid RAG cells. Heterokaryons containing complete genomes of both parental cells randomly lost chromosomes from both species. The majority of cells in early growth stages, however, still possessed a nearly complete human genome. The rate of human chromosome loss in subsequent growth periods was not uniform, being gradual in some and rapid in others. The initially predominant 2n human-1s mouse (1h:1m) type was soon replaced by a less frequent 2n human-2s mouse (1h:2m) type. Over an increased period of time in mass culture, the number of stemlines decreased. One stemline, often a (1h:2m) type with a greatly reduced human complement, outgrew the others and occupied the entire culture. Therefore, the usual process of clonal isolation may confer a negative selection bias against cell hybrids retaining a large number of human chromosomes. Hybrid stemlines with stable karyotypes were established in the present HAT-agar selection system before 36 days after fusion had elapsed.
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