To maximize the productivity of tissue plasminogen activator (TPA) by a mammalian cell culture, on-line determination of the optimum time to switch from the cell growth phase to the TPA production phase was investigated. By measuring the TPA production activity of the cells during the cell growth culture, it was shown that this optimum time was not necessarily the same as the time at which the cell concentration was maximized, and that the optimum time varied with growth culture batch. The TPA production activity of the cells during the growth culture could be estimated by on-line regression analysis using physiological data of the current state, including the oxygen consumption rate (Io(2)) and cell concentration, as well as data from pastatches. Applying this on-line estimation, the optimum switching time was determined to be the time at which the TPA production activity of the cells in the growth culture became highest, or higher than a certain value according to determined criteria.