Abstract— Ultraviolet B radiation (280-320 nm) can systemically suppress contact hypersensitivity (CHS), delayed type hypersensitivity (DTH) and tumor rejection responses in mice. Several models have been postulated for the initiation of this UVB-induced immune suppression and, although the complete mechanism is unclear, our early studies suggested that initiation is via the activation of a photoreceptor in the skin, identified as urocanic acid (UCA). Recent preliminary data from our laboratory and others indicated that UVA (320-400 nm)-emitting broadband sunlamps can also isomerize UCA but may not lead to immune suppression, in contrast to UVB-emitting sunlamps, which cause both effects. Although the reason for this inconsistency is unknown, the emission spectra of UVA lamps contain differing amounts of UVB, UVA-I (340-400 nm) and UVA-II (320-340 nm) from those of UVB sources. In this study we determined a detailed dose-response for the isomerization of UCA in mouse skin using the UVA-I, UVA-II and UVA-I+II wavelength ranges. The dose-response curves obtained were put on an equal energy basis by quantum correction and the possibility of wavelength interaction for this effect investigated. A simple additive wavelength interaction between UVA-I, UVA-II, and UVA-I+II was observed for trans-UCA photoisomerization. This result indicates that the failure of UVA-I, UVA-II or UVA-I+II radiation to induce immune suppression of the CHS response in an animal model is not due to complex wavelength interactions and/or the presence of an in vivo endogenous photosensitizer of UCA isomerization. Other factors, such as downstream blocking by UVA of the cis-UCA generated signal, may be involved.