H2 O and D2 O sorption isotherms have been obtained for air-dried and freeze-dried preparations of Bacillus megaterium spores using drying and equilibration techniques that resembled closely those employed in radiation experiments reported extensively with this organism. No significant difference exists between values of "apparent monolayer capacities" for spores prepared in these two ways and, at low values of <tex-math>$P/P_{0}$</tex-math>, between the adsorbed amounts, in molar terms, of the two kinds of water. For H2 O adsorption, the numbers of water molecules/DNA phosphate at "monolayer capacity" and at saturation vapor pressure are calculated to be 1.2 and 4.5, respectively. These values can be related to radiation sensitivity of the spore.

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