Skin protein modification (haptenation) is thought to be a key step in the manifestation of sensitization to low molecular mass chemicals (<500 g/mol). For sensitizing chemicals that are not protein reactive, it is hypothesised that metabolic activation can convert such chemicals into protein reactive toxins within the skin. trans-Cinnamaldehyde, alpha-amyl cinnamaldehyde, and trans-cinnamic alcohol are known sensitizers with differing potencies in man, where the former two are protein reactive and the latter is not. Here, we have used immunochemical methods to investigate the extent of protein-cinnamaldehyde binding in rat and human skin homogenates that have been incubated (for either 5, 15, 30, or 60 min) at 37 degrees C with cinnamaldehyde, alpha-amyl cinnamaldehyde (at concentrations of between 1 and 40 mM), and cinnamic alcohol (at higher concentrations of 200 or 400 mM). Cinnamaldehyde specific antiserum was raised specially. A broad range (in terms of molecular mass) of protein-cinnamaldehyde adducts was detected (as formed in a time- and concentration-dependent manner) in skin treated with cinnamaldehyde and cinnamic alcohol but not with alpha-amyl cinnamaldehyde. Mechanistic observations have been related to relative skin sensitization potential, as determined using the local lymph node assay (LLNA) as a biological read-out. The work presented here suggests that there is a common hapten involved in cinnamaldehyde and cinnamic alcohol sensitization and that metabolic activation (to cinnamaldehyde) is involved in the latter. Conversely, there does not appear to be a common hapten for cinnamaldehyde and alpha-amyl cinnamaldehyde. Such mechanistic work on protein modification is important in understanding the early mechanisms of skin sensitization. Such knowledge can then be used in order that effective and appropriate in vitro/in silico tools for predicting sensitization potential, with a high confidence, can be developed.