There have been many unsuccessful attempts to induce gametocytogenesis in vitro. In the present experiment, however, we found that RPMI-CS medium and RPMI-FS medium prepared by dissolving powdered RPMI 1640 medium in the culture supernatants of hybridoma cells, hybrid line D21 and 219.5, respectively, that produce anti-P. falciparum antibody induced gametocytogenesis. Gametocytogenesis was consistently observed from 3 days after addition of these media. The culture supernatant of anti-P. falciparum antibody producing hybridoma cells did not induce gametocytogenesis in the absence of RPMI 1640 medium. RPMI-MS medium, prepared by dissolving powdered RPMI 1640 medium in the culture supernatant of myeloma cells, SP2/O-Ag 14, which was used as a control, induced a few gametocytes.