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Merozoites of Plasmodium falciparum depend on glycophorins for invasion into human erythrocytes, although this dependency varies between different geographic isolates of the species. The FCR-3 (Gambia) isolate appears to be fully dependent on the N-acetylneuraminic acid (NeuNAc) residues of the O-linked tetrasaccharide of glycophorin for invasion. Invasion of the CDC-1 (Honduras) isolate into neuraminidase treated erythrocytes is 50% of that into normal erythrocytes. This and additional results suggest that this isolate is not fully dependent on the O-linked saccharides of glycophorin. In the present study, invasion of CDC-1 and FCR-3 isolates into erythrocytes was examined in the presence of Fab fragments of monoclonal antibodies directed against different domains of glycophorin. Fab fragments directed against the carbohydrate domain inhibited invasion of both isolates but inhibited invasion by the FCR-3 isolate more than CDC-1 isolate. The reactivity of monoclonal antibodies (Mabs) directed against the carbohydrate domain was dependent on the NeuNAc residues as binding was abolished in neuraminidase treated erythrocytes. Mabs directed against the peptide domain of glycophorin A did not significantly inhibit invasion by either isolate. These results are consistent with other findings that the CDC-1 isolate is not fully dependent on the carbohydrate domain of glycophorin for invasion.