Abstract: Routine immune function testing in preclinical drug development was established as a regulatory requirement in June of 2000 under the Committee of Proprietary Medicinal Products (CPMP) Note for Guidance on Repeated Dose Toxicity (CPMP/SWP/1042/99). The purpose of the more stringent approach to immunotoxicology testing was to better identify unintended immunosuppression; however, the requirement was met with much discussion and debate. At the center of the discussion was an attempt to reconcile opposing regulatory directives from agencies outside of Europe that adhere to a more selective, weight-of-evidence approach to functional evaluations. Uncertainty over the predictive value of the recommended immune function tests relative to conventional toxicology parameters prompted an investigation by the International Committee on Harmonization (ICH). The results of a preliminary, industry-wide survey indicated that only a low percentage of pharmaceuticals adversely affect immune function without alterations to standard toxicology parameters. Expected ICH guidelines will ultimately determine to what extent and for what purpose immune function tests will be conducted. In the meantime, optimization of the recommended immune function tests is ongoing. The T-cell dependent antibody response (TDAR) by either conventional Sheep Red Blood Cell (SRBC) plaque assay or by the modified ELISA method using either SRBC or keyhole limpet hemocyanin (KLH) as antigen is being extensively evaluated to determine best practices and procedures for preclinical immunotoxicity evaluations. This review addresses some aspects of the debate concerning the appropriateness of immune function tests for hazard identification, along with recommendations for optimizing TDAR methodology to ensure adequate sensitivity and predictability in risk assessments for immunotoxicity.