Several murine monoclonal antibodies were raised against purified soluble Plasmodium falciparum antigens from the asexual blood stage. The monoclonal antibodies were purified from ascites by preparative agarose gel electrophoresis and tested for inhibitory activities against P. falciparum in vitro cultures. One monoclonal antibody, HATR 2–4, showed an isolate-specific growth inhibition of P. falciparum in vitro cultures. The antibody reacted in immunoblotting with bands of 250 and 57 kilo dalton (kdalton). Another monoclonal antibody, HATR 2–8, showed growth inhibition of several geographically distinct P. falciparum isolates. HATR 2–8 reacted in immunoblotting with bands of 250 and 74 kdalton. Heating of the antigens destroyed the reactivity of HATR 2–8. The monoclonal antibodies HATR 2–4 and HATR 2–8 probably recognize different epitopes on the same antigen. This antigen circulates in the plasma of some patients with P. falciparum parasitaemia.