Monoclonal antibodies (mAbs) are an effective tool in therapeutics and diagnostics. A novel approach called the single-cell RT-PCR-linked in vitro expression system (SICREX) enables the high-throughput generation and screening of mAbs from single B cells. In this paper, instead of using B cells, cDNAs were synthesized from single plasma cells of an immunized mouse spleen. The light chain (Lc) and the I'd portion of the heavy chain (Hc) genes of each cell were amplified separately and followed by overlapping PCR to add a 17 promoter, a ribosome-binding site, and a 77 terminator. The paired Lc and Hc genes were simultaneously expressed by an Escherichia coli in vitro transcription and translation system followed by ELISA to measure their affinity for the antigen. A Fab fragment with affinity against the antigen was obtained from plasma cells of an immunized mouse with hepatitis B surface antigen (HBsAg). (C) 2009, The Society for Biotechnology, Japan. All rights reserved.