THE first identifiable member of the B-cell line, termed the pre-B-cell, has been shown by immunofluorescent techniques to contain small amounts of intracellular IgM, but to lack detectable surface immunoglobulin1. These cells have been found in several species at sites of B-cell generation, most notably fetal liver and adult bone marrow1,2 and considerable evidence now exists which supports this precursor-progeny relationship of pre-B cells and B lymphocytes1-5. However, conflicting data exist with regard to pre-B cell expression of μ and light chains. Although these cells were originally described in mice as containing both intracellular heavy (μ) and light chains1, using the same immunofluorescence techniques we have subsequently been unable to demonstrate either κ or λ light chains within these cells (P.B. and J.K., unpublished). Moreover, the presence of light chains in murine pre-B cells cannot be confirmed with the original anti-κ antibodies used in ref. 1 (M. Cooper, personal communication). These findings are also contrary to immuno-chemical data which suggest the presence of complete IgM molecules both intracellularly and on the surface of early fetal liver cells6. We have used somatic cell hybridisation to approach this controversial issue. We have now obtained, after fusion with fetal liver, hybridomas which synthesise large amounts of intracellular μ chain in the absence of any light chain (LC) synthesis detectable by immunofluorescent or immunochemical techniques. The immunoglobulin phenotype of these hybridomas is identical to that observed in immunofluorescence studies of pre-B cells from fetal liver or adult bone marrow, and suggests that synthesis of μ chain before light chain expression is a normal event in the early differentiation of the B-cell line. © 1979 Nature Publishing Group.