The present Key Event based Test Guideline addresses the human health hazard endpoint skin sensitisation, following exposure to a test chemical. Skin sensitisation refers to an allergic response following skin contact with the tested chemical, as defined by the United Nations Globally Harmonized System of Classification and Labelling of Chemicals (UN GHS). This Test Guideline is proposed to address the Molecular Initiating Event leading to skin sensitisation, namely protein reactivity, by quantifying the reactivity of test chemicals towards model synthetic peptides or amino acid derivatives containing either lysine or cysteine. This Test Guideline provides three in chemico test methods addressing the same Key Event on the Adverse Outcome Pathway for Skin Sensitisation: (i) the Direct Peptide Reactivity Assay – DPRA, (ii) the Amino Acid Derivative Reactivity Assay – ADRA and (iii) the kinetic Direct Peptide Reactivity Assay – kDPRA. The DPRA and ADRA are used for supporting the discrimination between skin sensitisers and non-sensitisers in accordance with the UN GHS. In contrast, the kDPRA allows discrimination of UN GHS subcategory 1A skin sensitisers from those not categorised as subcategory 1A, i.e. subcategory 1B or no category but does not allow to distinguish sensitisers from non-sensitisers.