Optimization of culture conditions such as the dissolved O-2 (DO) concentration, temperature and pH was attempted regarding both cell growth and the production of tissue plasminogen activator (TPA) in a microcarrier cell culture of human embryo lung cells. The growth rate was suppressed at a DO concentration below 30% saturation. From the pH range 7.2-7.6, both the specific growth rate and maximal cell concentration decreased. At a lower temperature than 37 degrees C, although both the specific growth rate and the maximal cell concentration decreased, the cell concentration was maintained for a longer time during the production period, high TPA productivity being maintained. As the optimal conditions for culture growth, a DO concentration of 30% saturation or over, temperature of 37 degrees C and pH of 7.4 are recommended. However, for TPA production after cell culture growth, the DO concentration should be in the range 20-30% O-2 saturation, and the temperature and pH should be lowered to 33 degrees C and 6.8, respectively.