Abstract: This invention provides a process for checking the removal rate of pyrogenic substances, in particular viruses, from organic material. The material to be purified is passed through an ultrafilter or an ultrafiltration unit whose virus-removal rate has already been determined by passing viruses of the Leviviridae family or other comparable small bacteriophages through the filter or filtration unit, the virus titer is determined before and after the filtration operation and the virus removal rate thus calculated. Before or after determining the virus-removal rate, a given pressure is applied to the filter or filtration unit in a gas pressure-hold test, and the decrease in the pressure over a given period is measured. After filtration of the biological material, the rate of virus removal by the filter is checked by repeating the pressure-hold test.