Photoprotection of the skin is mainly a function of clothing, although the effectiveness of the latter against UV-B solar radiation (wavelengths 290-320 nm) has not been measured in vivo. Since UV-B mediates the cutaneous formation of vitamin D3, we examined the attenuation of that photosynthetic reaction by the commonly used fabrics cotton, wool, and polyester in black and white colors. Direct transmission of UV-B was attenuated the most by black wool (98.6% of incident irradiance) and the least by white cotton (47.7%). None of the fabrics allowed the photoproduction of previtamin D3 from 7-dehydrocholesterol irradiated in vitro with up to 40 min of simulated sunlight or the elevation of serum vitamin D3 after irradiation with approximately one minimal erythema dose (MED) of UV-B in volunteers wearing jogging garments made of these fabrics. Increasing the whole body irradiation dose to six MEDs still failed to produce a serum vitamin D3 response in garment-clad subjects. Regular (seasonal) street clothing also prevented an elevation of the vitamin D3 in response to UV-B radiation. We conclude that clothing prevents or significantly impairs the formation of vitamin D3 after photostimulation with up to six MEDs of UV-B.