Employers use drug testing for pre-employment testing of job applicants, for-cause testing of employees, and random testing of employees. Drug testing methods are predominantly the urine-based bioassays, typically used for screening; and gas chromotography-mass spectrometry (GC-MS) to confirm any positives from the initial screens. The main impetus for the widespread adoption of drug testing was the passage of the Federal Drug Free Workplace Act in 1988 and the subsequent promotion by the Federal Government of drug testing in the workplace. A report from 1994 shows that adults employed full-time constituted 69 percent of all illicit drug users aged 18 to 49. Marijuana is by far the most commonly used drug. Although a variety of studies have attempted to assess the impact of drug testing on the work force, evidence of its preventive/deterrence effects are not clear. In the first phase of this study, 800 employees were randomly selected from a work force in a large steel plant. The purpose was to compare different methods of self-assessment to each other and to bioassay techniques, and to explore the combining of methods to produce a more accurate prevalence estimate. Results showed that drug-use prevalence in a workplace was likely to be 50 percent higher than the estimate based on self-reports. The second phase of the study used 340 workers and compared self-reports and two types of bioassay data on current and recent use of illicit drugs. Results showed dramatic differences in drug-use rates by the different assessment methods. The need for combining data from both self-reports and bioassays in order to produce more accurate estimates of drug use appears to be supported by these findings. 23 tables, 25 references