Upon exposure to UVB, the epidermal component trans-urocanic acid is not only photoisomerized into cis-urocanic acid, but will also, at least in part, be photooxidized into urocanic acid oxidation products. We hypothesized that urocanic acid oxidation products can mimic UV-induced systemic immunosuppression comparable to the suppressive properties already established for cis-urocanic acid. A crude mixture of urocanic acid oxidation products showed a significant suppression of the sensitization phase of the systemic contact hypersensitivity response to picryl chloride. Three of the urocanic acid oxidation products were selected for this study: imidazole-4-carboxylic acid, imidazole-4-carboxaldehyde and imidazole-4-acetic acid. Effects on the sensitization-, elicitation- and post-elicitation phase of contact hypersensitivity to picryl chloride in BALB/c mice were studied and compared to the effects of cis-urocanic acid. Imidazole-4-carboxaldehyde was equally effective at suppressing the sensitization phase as cis-urocanic acid. The triplet combination of the imidazoles showed more pronounced suppression than that induced by cis-urocanic acid. The most effective compounds for the suppression of the elicitation phase appeared to be imidazole-4-acetic acid and cis-urocanic acid. Significant suppression of the post-elicitation phase was only obtained with the triplet combination of imidazole-4-carboxaldehyde, imidazole-4-carboxylic acid and imidazole-4-acetic acid, which combination appeared to be effective at all three tested phases, Because these three urocanic acid oxidation products are present in UVB-exposed human stratum corneum, these compounds may play a role in UVB-induced immunosuppression.