In vitro infection with Epstein-Barr virus of bone marrow-derived (B) lymphocytes from blood of patients with X-linked or common varied agammaglobulinemia resulted in the establishment of long-term B-lymphoid cell lines (LCL). LCL were established only with B lymphocytes from patients whose B cells failed to respond to in vitro mitogenic stimulation. In contrast, lymphocytes from patients without small B lymphocytes or with B cells that synthesized but failed to secrete Ig failed uniformly to establish LCL. Analysis of the "nonsecretory" B lymphocytes demonstrated the presence of receptors for C3b and C3d as well as surface Ig, but absence of detectable receptors for Epstein-Barr virus. All of the six cell lines that were established formed rosettes with EAC3 but not with sheep erythrocytes. Four of the six LCL were principally surface IgD bearing and did not synthesize detectable Ig for secretion. Two of the cell lines were indistinguishable from cell lines from normal individuals: they had surface IgG, A, M, and D and synthesized and secreted Ig.