The influence of alterations in the format of antibodies on their specificity has been examined. To analyze the role of Ig constant regions in recognizing antigens, a comparison was made of the specificities of full-scale murine monoclonal antibodies and scFv single-chain miniantibodies obtained from the latter with regard to a group of closely related protein antigens - Staphylococcus enterotoxins. It was found that in the scFv format the specificity and affinity of miniantibodies diminished as compared to the full-scale ones. Specificity of antibodies may be enhanced by transforming them into full-scale antibodies. Moreover it was shown that miniantibodies within a phage particle generated from combinatorial phage libraries possess greater specificity to the antigen, however during the subsequent transformation to soluble scFv antibodies their specificity diminishes. © 2011 Elsevier Ltd.