Malaria transmission requires that Plasmodium parasites circulating in the vertebrate host develop into male and female gametocytes, which are then taken up by a mosquito to undergo fertilization and further development into infectious sporozones. To understand the malaria specific events involved in this process, the gene products involved require identification and characterization. This work demonstrates that antibodies generated against the paralog of malaria transmission-blocking antigen Pfs230, PfI30400w, react only with stage V male gametocytes, not gametes or asexual parasites. In contrast. Pfs230 is expressed on the surface of all gametocytes and remains associated with emerged gametes as one of the primary surface antigens for several hours. Consistent with the localization findings, a high molecular weight band is recognized by anti-PfB0400w antibodies on western blots of extracts of late stage gametocytes, not asexual parasites, early (stage II/III) gametocytes, or gametes. PfB0400w mRNA is also not observed in asexual parasites. The transcript levels peak in stage III/IV gametocytes, then sharply decline in gametes. This work identifies a novel male-specific protein with an expression pattern that is distinctly different than its paralog. (C) 2002 Elsevier Science B.V. All rights reserved.