When exposed to relative humidity of 80%-90%, human and animal hair has been shown to be rapidly attacked by airborne microorganisms. Damage includes exfoliation of cuticular scales, distortion of the cortex and medulla, dramatic color changes in some samples, and eventually, complete disintegration of portions of the hair shaft. Degreasing with diethyl ether briefly delayed the onset of attack, but had no other discernible effect on the process of biodeterioration. The hair was severely damaged long before the presence of microorganisms could be detected with the unaided eye. These findings suggest that museum collections of mammals, especially those from tropical environments, should be stored at relative humidities below 50% to prevent regeneration of mold spores.