Disclosed herein is a cell harvesting instrument suitable for concentrating cells from a source suspension of cells and/or washing said cells, the instrument comprising: a housing for accommodating mechanical elements including at least one fluid pump, at least one valve; and a processing kit removably insertable into the housing, said kit including a generally flat frame having or supporting plural sealed fluid paths arranged in a generally flat plane and such that fluids in the paths do not contact said mechanical elements, wherein at least portions of the fluid paths comprise flexible tubes, the outer surfaces of which are manipulatable by the or each fluid pump, to provide fluid flow in one or more of the paths and/or by the or each valve to restrict fluid flow in one or more of the paths. In an embodiment, the kit comprises also a fluid processing reservoir and a filter suitable for separating cells from fluid in said paths. A transfer mechanism for moving and weighing the fluid processing reservoir is disclosed also.