The popliteal lymph node assay (PLNA) in mice represents a predictive test for assessing the sensitizing (allergenic and autoimmunogenic) potential of drugs and low molecular weight chemicals. Measuring activation of the draining lymph node of the hind paw, the PLNA facilitates the detection and analysis of immunotoxic effects in a rapid and reproducible manner. An attractive feature of the PLNA is that it can be performed in combination with the routine toxicity testing required for new drugs. Thus, it is possible to investigate whether animals exposed by the oral, intravenous, or inhalative route have been sensitized to the test compound or a reactive metabolite of the test compound generated in vivo. PLNAs may be appropriate supplements to routine toxicity screening of chemicals, thereby enhancing chemical safety.