Of the semiconductor metals, only arsenic has been extensively studied as a human carcinogen and systemic toxicant. Recent studies have shown, however, that gallium, arsenic, and indium are capable of producing marked alterations in cellular gene products. After acute in vivo administration indium and thallium have been shown to produce decreases in the activity of some drug-metabolizing enzymes dependent on cytochrome P-450; therefore these metals would be capable of interfering with the metabolism of organic carcinogens. Selenium is essential for the activity of the enzyme glutathione peroxidase, which modulates the active intermediates generated by drug-metabolizing enzyme systems. Germanium produces toxicity in a number of organ systems. Antimony produces lung and circulatory system effects. Overall, available data suggest that these metals or metalloids are capable of biologically altering several cellular defense mechanisms involved in the carcinogenic process and that further studies are needed to determine the associated risks.