Monoclonal antibody 4B7 is a neutralizing antibody that binds the protein Pfs25 in the sexual stages of the malaria parasite Plasmodium falciparum and completely blocks transmission of the parasite from human serum to the mosquito host. Here we report the identification of the epitope on Pfs25 recognized by 4B7 and the crystallization of the intact murine monoclonal antibody with peptides corresponding to that epitope. This study highlights the importance of ligands in the crystallization of proteins. In this case peptides have been used to modulate the solubility of the peptide-IgG complex and may have provided different or additional crystal contacts to create or enhance a crystalline reticulum. Multiple crystal forms characterize this crystallization and the various peptides, differing both in length and sequence, have been used to investigate how such changes affect nucleation and crystal growth.