Phage-display and competitive panning elution leads to the identification of minimum-sized antigen binders together with conventional antibodies from a mouse cDNA library constructed from HM-1 killer toxin neutralizing monoclonal antibody (nmAb-KT). Antigen-specific altered camelid-like single-domain heavy chain antibody (scFv K2) and a conventional antibody (scFv K1) have been isolated against the idiotypic antigen nmAb-KT. The objectives of the study were to examine (1) their properties as compared to conventional antibodies and also (2) their antifungal activity against different pathogenic and non-pathogenic fungal species. The alternative small antigen-binder, i.e., the single-domain heavy chain antibody, was originated from a conventional mouse scFv phage library through somatic hyper-mutation while selection against antigen. This single-domain antibody fragment was well expressed in bacteria and specifically bound with the idiotypic antigen nmAb-KT and had a high stability and solubility. Experimental data showed that the binding affinity for this single-domain antibody was 272-fold higher (K (d) = 1.07 x 10(-10) M) and antifungal activity was three- to fivefold more efficient (IC50 = 0.46 x 10(-6) to 1.17 x 10(-6) M) than that for the conventional antibody (K (d) = 2.91 x 10(-8) M and IC50 = 2.14 x 10(-6) to 3.78 x 10(-6) M). The derived single-domain antibody might be an ideal scaffold for anti-idiotypic antibody therapy and the development of smaller peptides or peptide mimetic drugs due to their less complex antigen-binding site. We expect that such single-domain synthetic antibodies will find their way into a number of biotechnological or medical applications.