A method is described for separating, retrieving and concentrating platelets from whole blood relying on aggregation of the platelets followed by filtration. This method eliminates the need of a centrifuge for separating said cells from blood. To obtain cellular concentrates of platelets, blood is mixed with compatible agents that will aggregate cells while retaining contained growth factors. The resulting aggregates can then be separated from blood by filtration. If desired, the filter-captured aggregates are subject to a brief washing cycle where they are washed clean of residual aggregating agent, plasma, and red cells. Aggregates can then be partly or wholly deaggregated and the cells retrieved. The result is a suspension of cells and small aggregates with therapeutic levels of concentrated blood cells with included growth factors that are available for delivery to a wound site. A device that accomplishes the aforementioned process is also described.