The productivity of the continuous-flow, cell-free protein synthesis system (CFCF-system) is inferior compared to that in the cell-free batch system. By comparing the synthesized protein present between in the reaction chamber and in the fed-out solution in the CFCF-system, it appeared that much of the synthesized protein remained in the reaction chamber. Addition of a nonionic detergent, n-dodecyl beta-D-maltoside, to the feeding solution resulted in enhancement of the productivity in the CFCF-system which indicated that the permeability of the membrane to the protein might be one of the most influential factors affecting the productivity of the CFCF-system.