The drugs examined were amphetamines, barbiturates, cocaine, hallucinogens, marijuana, and opiates. Predictive value (positive) is the percentage of screenees with positive tests who actually have the condition being screened (true positives). Predictive value (negative) is the percentage of persons who test negative and have actually not used the drug being tested (true negatives). The sensitivity and specificity of tests were obtained from manufacturers' reports of the accuracy of their own assays, additional unblinded evaluations of urinary drug screening tests, and blinded proficiency testing conducted by the Centers for Disease Control and others. Estimates of the prevalence of illicit drug use among the working population in 1982 were obtained from a household survey conducted by the National Institute on Drug Abuse. Estimated predictive values (negative) were generally high, but the estimated predictive value of a positive test ranged from 1 percent for amphetamines to 100 percent for hallucinogens and only 38 percent for marijuana, the most prevalent drug. 1 table, 1 figure, and 28 references. (Author abstract modified)