A radioimmunoassay specific for the MOPC 21 kappa (K) myeloma chain of NSI and X63 myeloma x spleen cell hybrids was used to study light chain secretion in myeloma-hybrid lines. The M1 series of rat spleen cell x NSI mouse myeloma hybrid lines was chosen to illustrate the application of the radioimmunoassay for K chain quantitation and identification of K chain loss variants. Most of these lines secrete H (specific heavy), L (specific light), and K (myeloma kappa) chains, i.e., are HLK lines. Assays specific for rat L chain and mouse K chain showed that the ratio of L/K chain secreted by 6 different hybrid HLK lines ranged from 1.1 to 12.4. Using the rapid radioimmunoassay screening procedure, HL clonal variants which had lost K chain secretion were isolated at a frequency of approximately 10(-2) and characterized. K chain loss was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) of radiolabelled secreted products. Stability of one HL line and its HLK parent was examined during 9 months of growth in vitro. The HL line remained stable, while antibody secreted by the HLK line became inactive, apparently due to overgrowth by clonally dominant HK cells which no longer secreted specific L chains. The radioimmunoassay appears to detect MOPC 21 kappa chain variable region determinants. Therefore, although used here with rat-mouse hybrids, it should also be possible to use the assay to obtain mouse-mouse variant hybrid lines secreting antibody of improved homogeneity.