Abstract: A bioreactor system (1A; 1B) includes a variable volume container (3) having a flexible outer wall (27, 29) defining a cell-culture chamber for containing a fluid culture medium, and anchorage dependent cells in suspension. A agitator unit of the system (1A; 1B) is in fluid communication with first, second and third fluid ports (43, 45, 47A; 47B) for performing controlled replenishment of the culture medium in the cell-culture chamber, to admit fresh nutritive culture medium into the cell-culture chamber and for allowing draining of waste culture medium from the cell-culture chamber. The agitator unit further has a roller (15) for pinching the outer wall of the variable volume container (3) to thereby increase its a volume that forms the cell-culture chamber, starting from the initial minimum volume area (23), and means for gently agitating the cell-culture chamber.