Abstract: The present invention deals with a process and device for the separation of a body fluid from particulate materials in the fluid to be used for a chemical analysis of the fluid components. The process and device are in particular suitable for the separation of plasma from whole blood. Furthermore the present invention relates to a testing kit for the above mentioned separation and analysis purposes. The device has a collector membrane of defined capacity which intimately contacts the small pore side of a separator membrane having asymmetric pores passing through it permitting the application of a body fluid to the large pore side of the separator membrane.