Abstract: A membrane filter comprises a membrane provided with pores with a pore size between 5 nm and 50 .mu.m. The membrane may easily and reliably be processed with use of silicon micromachining through depositing a relatively thin membrane layer on a support, by use of a suitable vapour deposition or spincoating technique, after which perforations are made in the thin membrane layer, for example by etching with aid of photo-lithography or an imprint technique. Such a membrane is very well applicable for the separation of biological cells. A membrane with a thickness smaller than the diameter of the perforations is in particular suited for the separation of vulnerable cells and may be used as a leucocyte filter or as a blood plasma separator of blood concentrates. A microfiltration membrane according to the invention may very well be applied as a support for the deposition of a separation layer for ultrafiltration, gas separation or catalysis.