We have established lymphoid cell lines with diversity of immunoglobulin expression by infection of bone marrow cells of four patients with congenital agammaglobulinemia with Epstein-Barr virus. Immunofluorescent study revealed that the cells of many of these lymphoid cell lines had characteristics of possible precursor B cells, permitting classification as follows: 1) cell lines without any surface or cytoplasmic immunoglobulins (17/28 lines), 2) cell lines which had only cytoplasmic mu heavy chains (line K4 and O2), 3) cell lines with both mu and delta heavy chains in the cytoplasm (line O6 and M5), 4) cell lines which bore surface mu chains and possessed cytoplasmic mu chains but lacked light chain expression (line S5), 5) cell lines which had surface and cytoplasmic delta and lambda chains, and secreted the immunoglobulins (line K5), and 6) cell lines which had surface IgM and cytoplasmic mu and light chains, and secreted the immunoglobulins (5/28 lines). Biosynthetic studies with 14C-leucine confirmed the patterns of immunoglobulin expression found by immunofluorescent analysis. These cell lines may represent some stages of B cell development and provide useful information on the pathophysiology of congenital agammaglobulinemia.