Plasmodium knowlesi merozoites were prepared by the polycarbonate sieving method of Dennis, Mitchell, Butcher & Cohen (1975). Merozoite function was assayed by their attachment to and invasion of rhesus erythrocytes at 37 °C. The early merozoites from the culture chamber were the most invasive, although maximum numbers of merozoites appeared later. Merozoites were most stable when incubated at room temperature (23 °C). At 37 and 0 °C invasiveness rapidly declined to zero. Attachment was rapidly lost at 37 °C but was retained at 0 °C. Attachment was unchanged in the pH range 6·8–7·9 but invasion was reduced at pH 7·9. The presence of l-fucose, d-galactose, d-glucose, d-mannose, N-acetyl-d-galactosamine or N-acetyl-D-glucosamine did not reduce invasion. Attachment and invasion were greatly reduced or abolished by the presence of 2·5 mm EDTA or EGTA, by lactoper-oxidase-catalysed iodination of the merozoites, or by treatment of the merozoites with trypsin at a concentration of 1 μg/ml or greater for 10 min at 23 °C.