UVB radiation is a potent environmental carcinogen that not only causes mutations in the skin but also profoundly suppresses skin immune responses. Although this UVB-induced suppression of antitumor immunity has a key role in skin cancer development, the wavelengths within UVB causing greatest in vivo immunosuppression in humans are as yet unknown. We have identified a wavelength dependency for immunosuppression in humans across the UVB spectrum. We established linear dose–response curves for UV-induced local suppression of recall contact hypersensitivity responses at four wavelengths between 289 and 322 nm and found peak immune suppressive effectiveness at 300 nm and no detectable suppression at 322 nm within a physiologically relevant dose range.