Present-day catarrhines (old world monkeys and hominoids) lack Gal α1-3 Gal β1-4 GlcNAc-R structures ( α-galactosyl epitopes) and produce the corresponding anti-galactosyl antibodies (anti-gal), while platyrrhines (new world monkeys) and non-primate mammals possess α-galactosyl epitopes and lack anti-gal. Anti-gal is shown to inhibit Plasmodium falciparum growth in culture in a concentration dependent manner, probably by binding to α-galactosyl epitopes on merozoite surface molecules and causing complement mediated damage. A P. falciparum-like malaria parasite may therefore have selected for the inactivation of an α1-3 galactosyl transferase in catarrhines. The implications of the results for the development of clinical immunity to falciparum malaria are briefly discussed.