A recombinant cell line (NIH3T3 : pLtkSN) was made by infecting parental cells (NIH3T3) with a recombinant retrovirus (pLtkSN) encoding herpes simplex virus thymidine kinase (HSVtk) gene. The cells expressing HSVtk (NIH3T3 : pLtkSN) grew 2.3 times more than the parental cells (NIH3T3) in Dulbecco＇s Modified Eagles Media containing 10% (v/v) horse serum. The NIH3T3 : pLtkSN cells also showed a significant enhancement in the maximal cell concentration and the specific growth rate even at 2.5% serum concentration. The specific O-2 uptake rate of NIH3T3 was 2.1 times greater than that of NIH3T3:pLtkSN. Under both O-2-limited and O-2-unlimited conditions, it appears that HSVtk plays an important role in enhancing the growth characteristics of animal cells.