Based on PEPSCAN analyses, a peptide derived from the 25-kDa surface protein of P. falciparum sexual stages (LDTSNPVKT, amino acids 122-130) was recently shown to react with transmission-blocking monoclonal antibodies. Subsequently, a pinbound construct (FDDTDPIKK; six amino acids substituted) was designed that reacted 1000 times better with the transmission-blocking monoclonal antibody 32F81 than with the parent LDTSNPVKT peptide. While this construct was obtained through single amino acid replacement studies, it now is shown that the combined contribution of the various substitutions is necessary to give the total effect. Free peptides comprising LDTSNPVKT or FDDTDPIKK were shown to inhibit the interaction of the transmission-blocking monoclonal antibody with plate-bound peptides, when prepared by conventional synthesis procedures. Compared with peptides that contained LDTSNPVKT, similar levels of inhibition could be achieved with an average 500-fold lower molar amount of peptides containing the FDDTDPIKK sequence. Affinity constants of the peptides for the transmission-blocking monoclonal antibody ranged from 1.3 x 10(5), for peptides that contained LDTSNPVKT, to 2.6 x 10(8), for peptides that contained FDDTDPIKK. The structural relationship between the epitope of the 25-kDa surface protein and the peptides was demonstrated by competition experiments with the transmission-blocking monoclonal antibody 32F81. Again, peptides comprising the newly designed sequence FDDTDPIKK competed about 200 times better than peptides with the parent LDTSNPVKT sequence.