Several types of microfiltration membranes - polypropylene, cellulose diacetate (nominal pore size = 0.2 mum) and polycarbonate (pore size = 0.4 mum) - were tested for their microorganism retention ability. Suspensions of Pseudomonas diminuta and bovine red blood cells (RBC) were used for experimental purposes as representatives of microorganisms. Based on microscopy and microbiological evaluation it was demonstrated that individual cells of both types are able to cross a membranes that is generally rated as an aseptic one. A simple mathematical model is also proposed to describe the process by which the cells are transported across the membrane.