Lycopene is a carotenoid present in human blood (approximately 0.5 micromol/liter plasma), and the tissue levels vary from 1 nmol/g wet wt in adipose tissue to up to 20 nmol/g wet wt in adrenals and testes. Its biological activities include antioxidant activity (singlet oxygen quenching and peroxyl radical scavenging), induction of cell-cell communication, and growth control, but no provitamin A activity. Epidemiological studies suggest protective effects of lycopene on some types of cancer, e.g., prostate cancer. In vitro and in vivo studies on growth of tumor cells support this conclusion. The major sources of lycopene for the human are tomatoes and tomato products, and bioavailability from different food items varies considerably. Lycopene oxidation products have recently been identified in human serum. Suggested health effects of lycopene require further investigation.