A non-naturally occurring variant of a C-terminal fragment of a Plasmodium merozoite surface protein-1 (MSP-1) wherein said variant has (i) a reduced affinity, compared with a naturally occurring Plasmodium MSP-1.sub.19, for at least one first antibody capable of blocking the binding of a second antibody, which second antibody inhibits the proteolytic cleavage of Plasmodium MSP-1.sub.42 and (ii) substantially the same affinity for at least one third antibody compared with said naturally occurring Plasmodium MSP-1.sub.19. which third antibody inhibits the proteolytic cleavage of Plasmodium MSP-1.sub.42 is provided for use in an anti-malarial vaccine.