The expression vector pASK84 was designed for the convenient cloning of immunoglobulin variable domain genes, as well as periplasmic secretion of the corresponding F(ab) fragment in Escherichia coli. The plasmid provides the constant domain genes of mouse IgG1/kappa with a hexa-histidine tag fused to the C terminus of the heavy chain. This strategy enables the rapid and efficient purification of the functional recombinant F(ab) fragment via immobilized metal affinity chromatography. The versatility of this expression and purification system is demonstrated using the variable domains of the well-characterized anti-lysozyme antibody D1.3.