Humans are incapable of synthesizing the common mammalian cell surface sialic acid N-glycolylneuraminic acid (Neu5Gc), due to an irreversible genetic mutation that occurred after our last common ancestor with great apes. Despite this, we found trace levels of Neu5Gc in certain normal human tissues, and higher levels in fetal and malignant tissues, apparently due to incorporation from dietary sources. Circulating anti-Neu5Gc antibodies also occur in most normal humans, with marked individual variations in levels. We now show that while normal human blood cells metabolically incorporate very little free Neu5Gc from the culture medium, human leukemic cell lines do so efficiently, displaying it on their cell surfaces, as detected by flow cytometry using a monospecific polyclonal chicken antibody, and confirmed chemically by HPLC. Specific deposition of IgG from Neu5Gc-reactive human sera onto Neu5Gc-expressing leukemic cells could be demonstrated. This was associated with lytic cell killing, apoptosis, and antibody-dependent cell-mediated cytotoxicity. Human sera with low levels of anti-Neu5Gc antibodies did not mediate such effects. These data show for the first time that the “natural” anti-Neu5Gc antibodies found in normal humans can be functionally active. The selective incorporation of Neu5Gc into leukemic cells could provide an approach for targeting leukemic cells in vivo, via naturally occurring Neu5Gc-specific antibodies. However, in contrast to unstimulated blood cells, activated human T-lymphocytes also incorporated some Neu5Gc, albeit to a lesser degree than leukemic cells, allowing IgG binding and complement deposition. Thus, exposure of rapidly dividing activated T cells to Neu5Gc could potentially affect ongoing immune responses. Since incorporation into leukemic cells was most efficient, it may still be possible to define a therapeutic window to selectively target malignant cells.