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Hi Jan, In our lab we use both the Amicon Ultras and the Vivaspins (soluble and membrane proteins). What I can tell you is that me and a few other colleagues had the experience (and still do) of specific soluble proteins precipitating on the Cellulose membrane of the Amicon. By switching to the Vivaspins, which have a polyethersulfone membrane, the precipitation completely vanished. This seems to be very protein dependend as many seem to be able to use both systems with no or little problems. I don't know if the type of membrane really makes the difference but it could be a possible explanation. I unfortunately have less expierence with these systems on membrane proteins and I only know that both systems seem to work fine for colleagues who work on membrane proteins. Best Regards, Peter -- P.G.M. Gutte PhD student Institute of Biochemistry University of Zurich Winterthurerstrasse 190 CH-8057 Zurich Tel: +41 (0)44 635 55 02