The study of the synthesis, assembly, and secretion of IgM by seven murine myeloma tumors has revealed that free mu chain can be detected intracellularly after release from the ribosome. It combines with light chains to form microL. The major intracellular protein in six of the seven tumors was the 8S subunit. One tumor contained considerable amounts of 19S material intracellularly. Those tumors that did not contain 19S IgM intracellulariy appeared to assemble the subunits outside the cell.