A recombinant vector carrying rearranged immunoglobulin heavy (mu) and light (kappa) chain genes was introduced into several cell lines of non-B lineage including T cell (EL4) and two fibroblast lines (CV-1 and L cells). Our results indicated that intracytoplasmic mu-chains, both secretory and membrane types, were expressed in the recipient cell lines. Small amounts of kappa-chain transcripts, corresponding in size to authentic kappa-chain mRNA, were detected in EL4 and L cell transformants, whereas CV-1 transformants did not express the authentic kappa-chain RNA. EL4 and L cell transformants produced kappa-chain. This system will be quite useful for the analysis of regulatory mechanisms involved in immunoglobulin synthesis in non-B cells.