An established Chinese hamster cell line was fused with microcells isolated from phenotypically stable transferent mouse cells which contained a mouse transgenome coding for an abnormal form of mouse hypoxanthine phosphoribosyltransferase (HPRT, EC. No. 2.4.2.8) (Willecke et al. 1979). Two hybrids were isolated which expressed the abnormal form of mouse HPRT but no mouse alpha-galactosidase (GALA, EC. No. 3.2.1.22). In one of these microcell hybrids the abnormal HPRT activity segregated under counter-selective conditions with mouse chromosome 3. No mouse chromosome or additional mouse gene marker was found in the second microcell hybrid, possibly because of breakage and/or rearrangement of the integrated transgenome during the isolation of this hybrid. We conclude from these results that the transferred mouse HPRT gene is a phenotypically stable clone is not integrated at its homologous site on the host X chromosome. Rather, the transgenome is probably integrated into mouse chromosome 3, possibly due to homologies in repeated DNA sequences which may occur in the transgenome and which are interspersed at many sites in the host genome.