To overcome ammonia inhibition in animal cell culture, a method utilizing polyetrafluoroethylene membranes with an associated pH gradient was investigated. Model experiments without cells were carried out in three different reactor configurations suitable for animal cell culture processes to characterize mass transfer properties of the process. The actual fluxes of ammonia across the membrane ar e of the same order of magnitude as the typical ammonia production rate in an animal cell process, thus indicating the potential of the method for preventing the accumulation of toxic ammonia in cell culture.