The reticulocyte cell membrane was investigated for the role that it plays in cellular protein synthesis. It was found that a strong inhibition in protein synthesis occurred when a membranal extract was added to a cell-free protein-synthesizing system. The membranal extract was obtained by disrupting and then sulubilizing cell membranes with the non-ionic detergent Triton X-100. The inhibition in protein synthesis could neither be attributed to an RNAse-type action nor to a reduction in the energy level of the system. Preliminary results show that the principal site of action occurs at the elongation or/and termination stages of protein synthesis. Initial purification of the inhibitory component was achieved.