Quality control of any analytical method is integral to its ability to produce reliable results. To adequately control a method, one must first understand the sources of variability. Some sources of error in an immunochemical method are similar to those of a traditional chromatographic technique. Immunoassays also present some unique potential error sources along with novel opportunities for control of these errors. Many immunoassay parameters may be monitored including equipment performance and calibration, reagent lots, assay calibration features and quality control sample results. The effectiveness of a given parameter in predicting reliability or in diagnosing periodic problems should be understood. Once these features are understood, the immunoassay can be incorporated into a total quality control program which includes both an initial phase and routine phase. Statistical tests, graphic presentations, and control charts are powerful tools which enable one to effectively detect system errors.