Epstein-Barr virus transformation of B-cell-depleted bone marrow cells from human fetuses allowed us to identify novel cell types characterized by the expression of immunoglobulin κ or λ light chain without heavy chains. Four κ-only clones with normal karyotype were obtained and examined for their immunoglobulin gene configurations and expression. All four clones had κ-chain gene rearrangements at either one or both alleles, but the heavy-chain gene loci in these clones either were in germ-line context or had undergone only D-J(H) rearrangements (D and J(H) represent diversity and joining gene segments). All clones contained κ mRNA of normal size at levels consistent with the protein level, except for one clone that no longer produced κ protein. No μ mRNA or immunoglobulin heavy-chain molecules were detected in any of the κ+ clones. The results suggest that the μ heavy-chain protein is not an obligatory prerequisite for light-chain gene rearrangements.