Mature asexual parasites from cultures of knobby or knobless clones of P. falciparum containing 5 to 10% late forms were harvested by layering up to 10(9) erythrocytes on 3 ml Percoll. The density of the Percoll was adjusted to between 1.081 and 1.091 g cm-3, depending on the maturity of the parasites. Centrifugation at 1500 g for 10 min produced a sharp band at the interface containing mature parasites with a purity averaging 86%. Washed parasites derived from the layer or pellet showed good viability in vitro.