Approximately one-quarter of the ribosomes of P-3 mouse myeloma cells in tissue culture are bound to membranes of the endoplasmic reticulum. Incubation with NaF causes polyribosome disaggregation; at the same time approximately 25 % of the membrane-bound ribosomes are released from membranes. After reversing the inhibition of NaF free ribosomes become associated with membranes until approximately the same proportion of free and membrane-bound ribosomes found in untreated cells is reestablished. During this time these cells resume the normal synthesis and secretion of immunoglobulin. This takes place even when the synthesis of RNA is completely inhibited. These experiments have suggested that the association of a class of ribosomes with membranes is dependent on their entry into polyribosomes active in protein synthesis whereas attachment of the other ribosomes occurs independently of their participation in protein synthesis.