For an efficient production of retroviral vectors for gene therapy, the effect of serum concentrations in culture medium on the production and inactivation of retroviral vectors from the amphotropic producer cells (pMFG/ΨCRIP) was investigated in various cultivation methods. The serum concentration in the range of 1-10% did not show any significant influence on the inactivation of retroviral vector at 37 °C. The half-life of the vectors at 37 °C was in the range of 2.0-2.7 hours. However, the serum concentration significantly influenced the cell growth as well as vector production. In a commonly used method where the medium was exchanged once after the cells reached subconfluency, the maximum cell concentration and vector titer in 10% serum were 2.31 x 10 cells/cm and 6.2(±0.7; ±standard deviation) x 10 colony forming unit (CFU)/ml, respectively. On the other hand, the maximum cell concentration and vector titer in 1% serum were only 1.12 x 10 cells/cm and 1.3(±0.3) x 10 CFU/ml, respectively. Furthermore, a daily medium exchange did not enhance the cell growth and vector production in 1% serum. However, in 10% serum, a daily medium exchange allowed further cell growth and thereby enhanced the maximum vector titer (1.1(±0.I) x 10 CFU/ml). In addition, since it also improved the maintenance of high cell concentration, the vector could be produced throughout 16 days culture, suggesting the feasibility of perfusion culture system for continuous production of high titer retroviral vector.