The density of human erythrocytes infected in vitro with Plasmodium falciparum has been measured by isopycnic centrifugation in colloidal silica gradients. The densities of uninfected cells, rings, trophozoites, young schizonts, and mature schizonts were approximately 1.110, 1.110, 1.106, 1.097, and 1.090 g/ml, respectively. This information has been used to design a simple procedure for the separation of schizonts from other parasite stages and uninfected erythrocytes. By using synchronized cultures it is possible to obtain essentially pure schizonts after two centrifugations using a bench centrifuge. Such preparations are an excellent source of parasite antigen for immunological studies.