The cloning, expression and purification of a 32 kDa B. anthracis PA fragment (PA32) is described. This fragment has been expressed as a fusion construct to stabilized green fluorescent protein (EGFP-PA32). Both proteins bind to specific cell surface receptors. To confirm binding specificity, non-fluorescent PA83 or PA32 was used to competitively inhibit fluorescent EGFP-PA32 binding to cell receptors. The high intracellular expression levels and ease of purification make this recombinant protein an attractive vaccine candidate or therapeutic treatment for anthrax poisoning. Antibody fragments were isolated from a naive single-chain F.sub.v (scF.sub.v) library biopanned against PA83. Four scF.sub.v proteins were found to bind to PA83, the best one exhibiting a 10 nM K.sub.d. Two scF.sub.v proteins, scF.sub.v #1 and scF.sub.v #4, had similar affinities for PA32 and PA83, confirming the recombinant fragment was folded correctly.
 This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy to The Regents of The University of California. The government has certain rights in the invention.