We have studied the properties of 16 newly derived monoclonal antibodies (MoAbs) against Pfs230, a gamete surface protein of Plasmodium falciparum and a target of transmission-blocking antibodies. All 16 MoAbs recognized Pfs230 by immunoprecipitation from non-ionic detergent extracts of the protein radio-labelled with 125Iodine. The MoAbs also recognized this protein on Western blots under non-reducing conditions but none of them recognized the protein under reducing conditions. Using an immunoradiometric assay the MoAbs appear to define nine different epitope regions. The MoAbs were tested for their ability to lyse extra-cellular female gametes of P. falciparum isolate 3D7. Eight of the MoAbs, all of isotype IgG2a, mediated complement-dependent lysis of the gametes; seven of the MoAbs, all isotype IgG1, failed to lyse the gametes in the presence of active complement. The eight complement-fixing MoAbs mediated almost total suppression of infectivity of gametocytes of P. falciparum 3D7 to mosquitoes; where tested this suppression was mainly complement-dependent. The seven non-complement-fixing MoAbs had no significant effect on the infectivity of gametocytes of P. falciparum 3D7 to mosquitoes. When tested by immunofluorescence the target epitopes of all the MoAbs were conserved in each of the five different isolates of P. falciparum which were tested.