Mutants blocked or altered at different steps in the synthesis, assembly, or secretion of Ig by mouse myeloma cells have been isolated and are being characterized. Mutants have been isolated which synthesize heavy chains of altered molecular weight: these include mutants synthesizing chains with deletions in CH, or in the Fc and mutants synthesizing heavy chains 1.5 and 2 times the molecular weight of wild type. Mutants have been isolated which synthesize altered heavy chains which are assembled into HL half molecules instead of the complete H2L2 molecule. In addition, mutants have been identified which cease production of light chains; secondary variants which resume production of light chains have been identified. A genomic library has been cloned from the mouse myeloma 45.6 into Charon 4A and clones containing Ig sequences isolated. Cloning of the genome of selected mutant cells into Charon 4A is in progress.