Thank you very much for your answer, but I'm afraid this work won't help me. I ment that I want to isolate neutrophils from bags, which contain only buffy coats and not the whole blood. They can be purchased from blood donation station. I wonder i.a. what should be the dilution of such material.

Bags can sit for extended periods of time (often they don't release them until testing has been done and the are considered non-biohazardous), I tried and didn't get good results. The fresher the better, which can be tricky for ethics approvals etc, find someone is a nearby lab to donate, fresh blood dilute 1:1 RPMI, spin through Ficoll, use beads etc etc, keep everything cold post-ficoll and don't over pipette and agitate the cells.

Its quite tricky by Ficoll density gradient centrifugation. However, the layer beneath buffy coat (white in color) and above RBC layer may contain granulocytes, like neutrophils.