Technicians used five analytical procedures -- EMIT, TDX, RIA, TLC, and GC/MS -- to analyze urine specimens and determine whether or not certain classes of drugs were present. Specimens were tested for opiates, cocaine, phencyclidine, amphetamines, and marijuana. Each specimen was tested 25 times (5 tests for each of 5 drugs). Standard thin-layer chromatography was seriously deficient in the detection of all of the five drugs tested. When using the Federal guidelines for the establishment of cutoff levels for screening tests, none of the immunoassay technologies examined (EMIT, TDX, and RIA) was superior to the others in the detection of any of the five drugs. EMIT, TDX, and RIA were equally effective in the limitation of false positives for the substances tested. Confirmation of screened positives by a method based on a different chemical principle, preferably GC/MS, should be required if the subject contests the positive result or if one positive drug test will result in serious punitive action. 21 references, 7 tables, and appended supplementary information