AbstractSublines derived from V79 and B150 Chinese hamster cells lack hypoxanthine‐guanine phosphoribosyltransferase (HGPRT) and thymidine kinase (TK) respectively. When these strains are cultivated together without virus treatment in HAT medium, hybrid colonies with normal enzymic patterns arise in high frequency. The ratio of proliferating hybrid colonies to the minority cell inoculum in mixed cultures is approximately 1/1000. This observation has been used to examine the effect of x‐irradiation pretreatment of one cell type on the frequency of hybrid formation in mixed cultures. B150 cells (TK‐) were irradiated in monolayer cultures at dose levels of 200–4000r (180 KV, 15 ma). After trypsinization, the irradiated cells were mixed with unirradiated V79 cells (subline 129, HGPRT‐) and cultivated in HAT medium. The per cent decline in hybrid frequency as a function of x‐irradiation was compared to the relative decline in viability of x‐irradiated B150 cells when assayed in cultures alone. These curves diverged markedly, with the frequency of proliferating hybrids decreasing much less rapidly with radiation dose than survival of B150 cells. By contrast, when B150 cells were subjected to acute heat shock for graded intervals and subsequently mixed with untreated V79–129 cells, both viability of B150 cells and hybrid formation declined at the same rate. Our results suggest that lethally irradiated cells can still fuse with normal cells, and may contribute functional chromosomes to the hybrid karyotype. At limiting doses of x‐irradiation this technique may be useful as a means for induced segregation of m
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