The development of phage-display technology and the construction of huge libraries of antibody (Ab) fragments have provided an unlimited source of binders to virtually any antigen (Ag) (1). However, it is unlikely that the heavy (VH) and light (VL) chains of the Abs obtained from these libraries resemble original in vivo pairings. In certain autoimmune diseases and immunological processes, such as B-cell tolerance, these V_H and V_L combinations can be of crucial importance. To be able to determine the original V_H and V_L combinations of Abs, we have set up a single B-cell culture system. This comprises the sorting of individual lymphocytes into culture wells using flow cytometry, a culture system to expand these cells (2) and polymerase chain reaction (PCR) amplification of their variable-region genes, thereby immortalizing the V_H and V_L regions from individual human B cells. The method relies on the clonal expansion of single B cells in which cell-cell interactions (CD40-CD40L), as well as soluble factors, have been shown to be essential. One advantage beyond conventional hybridoma technology is that this method circumvents laborious plating and screening; the advantage compared to phage-display technology is that original V_H and V_L pairings can be isolated. This system has been used to analyze V_H and V_L pairings of human immunoglobulin G⁺(IgG+) B cells of unknown specificity (3) and, combined with a selection on the Ag U1A, a frequent autoantigenic protein target in patients with systemic lupus erythematosus, to analyze pairings in Ag-specific B cells (4). The efficiency of the system makes it possible to analyze large numbers of B cells and should therefore allow rare B-cell activities to be studied.