In this study we show that the room temperature multiwavelength autofluorescence signature of wet Bacillus endospores is considerably different from that of the dry endospores. The fluorescence is measured with excitation wavelengths of 300-500 nm with 50 nm steps. Small differences are observed between the dry and the re-dried spores. The fluorescence spectra support the model of reversible water migration between inner spore compartments and the environment. Characteristic information is extracted from spectral fluorescence excitation-emission contours for these spores by applying the dimensional reduction of data techniques known as principal component analysis. Aerosol detection and identification techniques relying on fluorescence must take into account the hydration state of the spores and recognize the ability of the spores to rapidly change their state of hydration. (c) 2004 Elsevier Ltd. All rights reserved.