Plasmodium knowlesi, a malaria of Old World monkeys, invades all Duffy blood group positive human erythrocytes and various New World monkey erythrocytes except Cebus apella. We had previously identified a 135 kDa parasite protein in supernatants of P. knowlesi cultures that bound to Duffy positive but not to Duffy negative human erythrocytes [Haynes et al., J. Exp. Med. 167, 1873-1881 (1988)]. We now use New World monkey erythrocytes as a reagent to identify P. knowlesi proteins in culture supernatants that will bind to all New World monkey erythrocytes susceptible to invasion but not to C. apella erythrocytes, which are refractory to invasion. The 135 kDa protein binds to all New World monkey erythrocytes, including C. appella. Another protein of 155 kDa binds to all New World monkey erythrocytes except C. apella. The 155 kDa protein binds to Old World monkey erythrocytes, the natural host of P. knowlesi; it does not bind to human Duffy positive erythrocytes. This and the previous study are the beginning of the identification of parasite proteins of P. knowlesi that bind to erythrocytes in a receptor specific manner.