1. The distribution of ribosomal components between the soluble and membrane fractions of a preparation of exponential-phase cells of Bacillus amyloliquefaciens lysed with lysozyme in 0.05m-tris buffer, pH7.6, containing 0.01m-Mg(2+), was strongly influenced by the addition of K(+) to the buffer, in the range 0-0.1m. 2. In the absence of K(+), 37% of the ribosomal material was bound to the membrane and was not removed by repeated washing with the lysing buffer. The amount of bound material was progressively decreased on increasing the K(+) concentration to 0.1m, when only 5% of ribosomal components remained attached to the membrane. 3. About 87% of the material that remained bound to the washed membranes prepared in the absence of added K(+) was removed on suspension of the membranes in a buffer containing 0.1m-potassium chloride. 4. In the absence of K(+), washed membranes, containing no detectable ribosomal material, were able to re-attach no more than half as much material as was found associated with membranes in the same buffer immediately after lysis. 5. There was no evidence of binding of specific components to the membrane. 6. In the presence of 0.05m-tris buffer, pH7.6, maximum incorporation of amino acids into protein by B. amyloliquefaciens polyribosomes is effected in the presence of 0.01m-Mg(2+) and 0.07-0.1m-K(+), under which conditions less than 10% of the ribosomal material of a cell lysate would be membrane-bound.