The cDNAs for the two variable domains of the antibody 9E10 were cloned from the hybridoma cell line. A chimeric 9E10 F a b fragment was produced in E. coli under control of the tightly controlled tetracycline promoter. The functional F ab fragment was isolated in a single step via a His 6 -tag, which also served for its recognition by a nickel chelatealkaline phosphatase conjugate. Thus, the recombinant F.,i, fragment permitted the immunochemical detection of the myc tag in a sandwich ELISA. The dissociation constant for the interaction with the myc tag peptide was determined as 80 ± 5 nM by fluorescence titration. In an attempt to produce the smaller 9E10 F v fragment it was found that its VH domain alone can be readily isolated from E. coli as a soluble protein. This unusual behaviour may be explained by the 18 amino acid-long CDR-H3 and could be of value in the design of 'single domain' antibodies.