The copper content of normal human cervical mucus varies during the menstrual cycle. It is lowest at midcycle when the copper content decreases sharply. When oral contraceptives are used, levels of copper in the mucus show a general increase, this being especially evident at midcycle when the normal decrease in levels is not observable. In addition, with oral contraceptive use, normal cyclic variation in the copper levels in mucus disappears. Copper content of the mucus is also noticeably higher in users of copper IUD's and slightly higher in users of plastic (non-copper-bearing) IUD's than in normal controls. Mucus samples from copper IUD users consistently have the highest copper content in all phases of the menstrual cycle. It is concluded that the presence of copper assists in preventing pregnancy.