Researchers often choose microbial tests for use in genetic toxicology studies because they are rapid and inexpensive, and because they supply dose-responsive data. This user's manual documents the test methods used extensively by the Health Effects Research Laboratory (HERL), Research Triangle Park, North Carolina over the past four years. The first part of the report describes the basic protocol (the plate incorporation test), special precautions, and specific modifications of the assay routinely used in HERL studies. The second part presents HERL's stepwise protocol for the plate incorporation test and protocols for several other biological assays, including the spot test, the forward mutation assay procedure, and the fluctuation test. The last portion of the report provides the necessary information for experiment planning and provides sample information forms and computerization procedures necessary for reporting experimental results.