The National Institute of Drug Abuse reports that 25 to 40 percent of applicants tested in certain occupations show recent illegal drug use, costing industry $50-$100 billion annually. Although reliability is the most frequently attacked aspect of drug testing, the kinetics of most drugs are understood and tests have been developed for them. The three most frequently used tests are discussed, and the cross-reactivity of such drugs as marijuana and ibuprofen and a lack of laboratory assessment are cited as problems. For positive results, specific followup tests in a reliable laboratory are recommended. The central question about the legality of drug testing is employees' right to privacy against searches under the fourth amendment. The article reports that no State or Federal Government prohibits testing by the private sector (only government intrusion is prohibited) and that reasonableness is the key issue in drug testing. The American Civil Liberties Union describes reasonableness as testing for reasonable cause to suspect drug abuse, such as bad performance or conduct. Helping employees who abuse drugs find treatment rather than firing them is recommended as more cost-effective than finding and training new employees. Photographs and one table.