To overcome ammonia inhibition in animal cell culture, a method utilizing porous polytetrafluoroethylene membranes with an assocd. pH gradient was investigated. Model expts. 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 are of the same order of magnitude as the typical ammonia prodn. rate in an animal cell process, thus indicating the potential of the method for preventing the accumulation of toxic ammonia in cell culture. [on SciFinder (R)]