We have previously described a new class of composite membrane that has the capability to efficiently remove particles, including viruses, from protein solution. The qualification of this membrane requires that it reproducibly and predictably remove model mammalian viruses. Using the Viresolve/70 membrane, the mammalian viruses polio, Simian virus-40, Sindbis, reovirus type 3 and Rauscher murine leukemia virus are shown to be reproducibly removed via a sieving mechanism. Mammalian virus retention increases constantly with virus diameter independent of virus class or type, increasing from 3.5 logs with polio virus to greater than 6.8 logs with murine leukemia virus. Consistent with a sieving mechanism, mammalian virus retention with the Viresolve/70 membrane is independent of virus concentration. These results are shown both in the presence and absence of protein in solution. The presence of protein in solution is shown to increase the virus retention coefficient of each virus above that measured in phosphate buffered saline. The model virus retention is shown to be well predicted by hard particle retention reported previously for this membrane. In addition, the hard particle retention is shown to predict the worse case performance expected of the membrane in the presence of protein.