Full mass spectral documentation together with a positive immunoassay and supporting data should insure the most certain identification. Preparation of specimens which test proficiency adequately is difficult, particularly for marijuana testing. The use of a full-spectrum GC/MS method should make the results admissible under the Frye Standard and the Federal Rules of Evidence, whereas ion monitoring methods could be challenged effectively. Duplicate specimens should be available, one for employer and the other for the employee. Precision suffers when determining low substance concentrations. 1 table and 11 references. (Author abstract)