Abstract: A method using diafiltration to prepare a blood product is described. The method removes contaminating plasma proteins from whole blood or blood products. Diafiltration may be used on paraformaldehyde-fixed platelets to remove formaldehyde to undetectable levels, and to suspend the platelets into a formulation buffer suitable for subsequent treatment, such as lyophilization. The manufacturing procedure can be divided into three steps, a first diafiltration, fixation, preferably with paraformaldehyde, and a second diafiltration. In the first step, plasma proteins are removed from fresh or stored platelets or blood cells. The technique is gentle to prevent the activation and aggregation of the platelets. Fixation stabilizes the platelets to withstand freezing, lyophilization and subsequent reconstitution, and also provides a viral and bacterial reduction.