Normal lymphoid cells obtained from hyperimmunized BALB/c mice and cells from most but not all IgG-producing mouse myeloma tumors and cell lines synthesized more light (L) than heavy (H) chains. While most myeloma tumors secreted their excess L chains, two tumors, MOPC 21 and MOPC 173, degraded the excess L chains and did not secrete them. A third group of tumors produced the same number of H and L chains. In the case of the MPC-11 tumor, excess L chains were synthesized by cloned tumors and cell lines.