After 10 months of continuous cultivation in vitro, two cell lines (HE3-2 and HE4-2 cell lines) of embryonic tissue origin from Syrian hamsters did not produce tumors upon subcutaneous implantation into the hosts. After 13 months, however, the similar implantation resulted in the development of rapidly growing transplantable fibrosarcomas. The tumor-producing capacity of HE3-2 cell line was higher than that of HE4-2 line. The chromosomal analysis of HE3-2 cell line after 10 months' culture in vitro showed hyperdiploid karyotypes (“B” karyotype) with modal value of 47, while that after 13 months showed the predominance of further altered karyotypes (“A” karyotype) with modal value of 45 to 46. On the other hand, the most population of HE4-2 cell line after 13 months in vitro contained “B” karyotypes and a minor part of it showed “A” karyotypes.