The in vivo inhibitory action of NIMP23, a monoclonal antibody raised against the rodent parasite Plasmodium chabaudi chabaudi AS, has previously been shown to be strain-specific, capable of delaying significantly the onset of P. c. chabaudi AS but not a P. c. chabaudi CB challenge parasitaemia. The epitope to which this mAb binds has been mapped to the second of two epidermal growth factor-like domains located at the C-terminus of the merozoite surface protein 1 (MSP-1) of P. c. chabaudi AS. The C-terminus region of the MSP-1 of P. c. chabaudi is a region of heterogeneity with AS and CB strain parasites showing only 78% identity at the amino acid level. The critical amino acid substitution which accounts for the strain specificity of the NIMP23 monoclonal antibody has now been identified. Polymerase chain reaction directed mutagenesis experiments demonstrate that a single proline to asparagine substitution at position 1722 in the primary amino acid sequence is sufficient to convert NIMP23-negative P. c. chabaudi CB expression constructs into NIMP23-positive clones whilst the converse substitution of an asparagine for a proline residue converts P. c. chabaudi AS expression constructs into NIMP23-negative clones.