A panel of Brazilian and Indian sera was screened for reactivity with a variant strain of Plasmodium vivax recently isolated in Thailand. This strain has been shown to have a unique repeat region which differs from the previously described P. vivax CS proteins. A total of 21/343 human sera were found to react with a synthetic peptide representing the variant P. vivax repeat. All of the sera that reacted with the variant repeat peptide, (ANGAGNQPG)4, also reacted with variant P. vivax sporozoites. Both the anti-peptide and the antisporozoite reactivity were totally abolished by adsorption with the variant peptide. Some of the human sera contained variant antibodies that were species specific and could only be adsorbed with the specific variant peptide. These findings suggest that the variant strain of P. vivax might have a worldwide distribution. We also found that some of the variant positive sera reacted with P. brasilianum sporozoites and with the P. brasilianum/P. malariae CS repeat. The adsorption of these sera with the P. brasilianum/P. malariae repeat peptide, (NAAG)4, significantly reduced the reactivity of these sera with the P. vivax variant. In addition, polyclonal and monoclonal antibodies of mice immunized with P. brasilianum sporozoites cross-reacted with the variant P. vivax CS. These findings suggest that exposure to P. brasilianum or P. malariae may give rise to sporozoite antibodies which cross-react with the P. vivax variant CS.