We have optimized primers for cloning libraries of murine heavy and light chain variable regions using the polymerase chain reaction. Since we are interested in cloning murine Fab fragments for expression in bacterial cells, the heavy chain primers were designed to clone Fd fragments comprising the heavy chain variable domain and the first domain of the IgG constant region. The light chain primers were designed to clone the entire murine kappa chain. Using ten degenerate 5' primers and a degenerate 3' primer to amplify murine Fd and seven degenerate 5' primers with a single 3' primer to amplify kappa chains, a diverse repertoire of mouse variable regions was cloned from mouse spleens.