Ultraviolet B (UVB) irradiation (285–320 nm) is considered important for metabolic processes and reproduction in many reptile species by facilitating the synthesis of vitamin D3. In captivity, UVB radiation reaching an animal may be diminished by the properties of the materials used in enclosure construction. We investigated the UVB-attenuating properties of 14 materials commonly used in cage tops for reptile enclosures. Irradiances were measured by two types of hand-held broadband radiometers and the D3-synthesizing potential was assessed by the use of an in vitro model. For UV-transmitting acrylic, a significant discrepancy between meter irradiances and in vitro model values for D3-synthesizing ability was observed, with meter readings underestimating the blocking effect. In contrast, attenuation of UVB irradiances by air-permeable materials, such as wire screen, measured with meters was generally comparable to the attenuation of D3-synthesizing ability as measured by in vitro models. Relatively simple meter readings can therefore be used to reflect reduction of D3-synthesizing ability through air-permeable materials. Zoo Biol 26:417–423, 2007. © 2007 Wiley-Liss, Inc.