In developing countries, neonatal tetanus causes significant mortality. Using a new competitive enzyme-linked immunosorbent assay to measure anti-tetanus toxin antibody levels, we compared rates of protection, total antibody levels, and maternal-to-fetal antibody ratios between different socioeconomic groups in Peru. Upper-middle-class women 25 years and older had significantly lower rates of protection and mean antitetanus toxin antibody levels than did lower-class women of the same age. Nevertheless, the former had higher fetal-to-maternal antibody ratios, independent of maternal age, total antibody levels, or parity. We conclude that future vaccination programs in Latin America must target upper-middle-class as well as lower-class women.