Percoll gradient centrifugation has been applied to Plasmodium falciparum- and P. yoelii-infected blood to allow rapid concentration of gametocytes. In the case of cultured P. falciparum, stage IV and V gametocytes were separated from immature stage II and III forms and concentrated from initial gametocytaemias of 1 to 3% to an average of greater than 80% and a maximum of 100% purity in a fraction which collected at the 1.064/1.083 g/ml interface; the rounded gametocytes of P. yoelii-infected mouse blood were also concentrated in this fraction. A much larger enrichment in microgametocyte concentration (average of 160-fold) was recorded in a fraction that collected at the 1.047/1.064 g/ml interface. The large increases in gametocyte concentration were paralleled by similarly large increases in exflagellation activity, thus confirming the viability of both preparations.