The basic principle underlying the Local Lymph Node Assay (LLNA) in mouse is that sensitizers induce a primary proliferation of lymphocytes in the auricular lymph nodes draining the site of chemical application. This proliferation is proportional to the dose applied and provides a measurement of sensitisation. The method described is based on the use of radioactive labelling to measure cell proliferation. A minimum of four animals is used per dose group, with a minimum of three concentrations of the test substance, plus a negative control group treated with the vehicle only, and a positive control, as appropriate. The experimental schedule of the assay is during 6 days. Thereafter, the animals are killed and a cell suspension of lymph node cells is prepared. The incorporation of 3H-methyl thymidine is measured by ¦Â-scintillation counting as disintegrations per minute (DPM). The Test Guideline includes performance standards that can be used to evaluate the validation status of new and/or modified test methods that are functionally and mechanistically similar to the LLNA. A reduced LLNA approach which could use up to 40% fewer animals is also described as an option. This study includes: measurements (weighing, DPM), and clinical daily observations. Results are expressed as the Stimulation Index (SI).The SI is obtained by calculation and should be ¡Ý3 before classification of the test material as a skin sensitizer is warranted.