Embodiments of the invention relate to a Disposable Bioprocess System including a Single-Use-Bioreactor, a Single-Use-Pump and a single-use micro-organism retention filter. Combined most suitable for cultivation of suspended micro-organisms in a liquid media at high micro-organism concentration in a perfusion mode continuous process for expression of biological material. The inlet port of the liquid Single-Use-Pump connects via a valve to the broth reservoir of the Single-Use-Bioreactor through a liquid conveying port. The outlet port of the liquid pumping Single-Use-Pump connects via a valve to a micro-organism retention filter. And a method for operating said sterile Disposable Bioprocess System in perfusion mode for continuously processing.