Plasmodium falciparum growth in vitro related to the concentration of inactivated, non-immune human serum supplement to the RPMI medium. This study investigated the concentration of non-immune serum required to support adequate in vitro parasite growth without saturating the medium. Parasitaemia was highest with 7.5% serum concentration in suspension cultures. However, peak parasitaemia obtained under static cultures with 12.5% serum concentration did not significantly differ from the level attained with suspension cultures at the same serum concentration. Ten per cent serum-supplemented medium supported parasite growth in static and suspension cultures, and levels of parasitaemia declined with further increases in serum concentration.