Ig synthesis was investigated in a human Epstein-barr virus transformed pre-B-like lymphoblastoid line, Josh 4. Translation of total extracted cellular RNA in a wheat germ cellfree system showed that the transcripts for both micrometer (the heavy chain of membrane IgM) and micros (the heavy chain of secreted IgM) were present in approximately equal proportions. Pulse-labeling of cells with 35S-methionine in culture followed by immunoprecipitation and endo-N-acetylglucosaminidase digestion of the precipitates demonstrated that each of these chains were synthesized and glycosylated intracellularly. No light chain synthesis was detected in either system. In addition, both mu-chains were found to be secreted. Hybrids formed between Josh 4 and 2 light chain-producing lymphoblastoid lines, RPMI-8866P (IgG kappa) and 32a1 (IgA lambda), were found to express surface IgM. Thus the provision of light chains permitted membrane expression of chains which might otherwise be degraded. These findings were discussed in relation to normal B cell development.