Tensile properties of Attacus atlas (Saturniidae) silk were measured in air, water, and ethanol. Control samples in air showed a large variability, so a normalization method was developed to enable comparison with the behavior of samples submerged in the liquid media. Amino acid analysis demonstrated that the composition of A. atlas silk is similar to that of silk produced by other members of the Saturnidae family. The tensile properties of A. atlas silk resemble those of Bombyx mori (common domesticated silkworm) silk, although the two silks have different compositions. In particular, the elastic modulus is decreased by immersion in water and increased slightly by immersion in ethanol. The behavior of A. atlas silk can be described in terms of water having a disruptive effect on protein-protein hydrogen bonds, whereas ethanol acts as a desiccant and therefore enhances protein-protein hydrogen bonding. There are significant differences between the tensile properties of A. atlas silk and spider (Nephila madagascarensis) drag line, even though these materials have similar amino acid compositions: the spider silk has previously been shown to exhibit a decreased elastic modulus and conspicuous supercontraction in both liquids. Primary composition is therefore not a simple indicator of mechanical function in silks.