Cholera toxin and the related Escherichia coli heat-labile enterotoxin are hexameric proteins comprising one A-subunit and five B-subunits. In this paper we report the generation and characterization of a monoclonal antibody, designated LDS47, that recognizes and precipitates in vivo assembly intermediates of the B-subunit (EtxB) of E. coli heat-labile enterotoxin. The monoclonal antibody is unable to precipitate native B-subunit pentamers, thus making LDS47 a useful probe for studying the early stages of enterotoxin biogenesis. The use of LDS47 to monitor the in vivo turnover of newly synthesized B-subunits in the periplasm of E. coli demonstrated that (i) the turnover of unassembled B-subunits followed an apparent first order process and (ii) it occurred concomitantly with the assembly of native B-pentamers (k = 0.317 ± 0.170 min-1; t( 1/2 ) = 2.2 min). No other proteins were co- precipitated with the newly synthesized B-subunits; a finding that implies that unassembled B-subunits do not stably associate with other periplasmic proteins prior to their assembly into a macromolecular complex. The use of overlapping synthetic peptides corresponding to the entire EtxB polypeptide demonstrated that the epitope recognized by LDS47 is located within the amino-terminal decapeptide of the B-subunit. From the x-ray structural analysis of the toxin (Sixma, T., Kalk, K., van Zanten, B., Dauter, Z., Kingma, J., Witholt, B., and Hol, W. G. J. (1993) J. Mol. Biol. 230, 890- 918), this region appears to resemble a curved finger that clasps the adjacent B-subunit. Thus, this region might be expected to be exposed in the unfolded or unassembled subunit, but to become partially buried upon assembly and thus inaccessible to recognition by the monoclonal antibody.