The effect of copper ions on the motility of human sperm was studied in vitro to determine whether copper is toxic to sperm. Sperm samples from 30 men of proven fertility who had high quality sperm were collected, separated from the seminal fluid, and resuspended in Menezzo, a chemical medium designed to resemble the secretions of the Fallopian tubes. Menezzo contains all the known constituents which aid sperm capacitation, including mineral salts, organic compounds, free amino acids, and enzymes. Its pH is between 7.4 and 7.6. After 1 hour of incubation the dead and weak sperm were separated out and only mobile sperm were tested. Crystallized copper sulfate was added to 4 of the 5 sperm-Menezzo mixtures. The final concentrations were of 1 millimolar, .1 millimolar, .01 millimolar, and .001 millimolar. Sperm motility showed a statistically significant decrease in the concentration of 1 millimolar but only after a latency period of at least 5 hours. At lesser concentrations and in shorter time periods no significant difference was observed compared to the control. The copper released from a copper IUD into the uterine fluid does not exceed a concentration of .1 millimolar. The lower concentration and the long latency period before the toxic effect manifests itself, seem to indicate that copper has no direct contraceptive effect. Copper is however known to promote inflammatory reactions, and it may work in vivo through the intermediacy of the leukocytes in the uterine cavity, either by directly stimulating phagocytosis or by release of substances toxic to sperm.