Using linewidth and spinning sideband intensities of lipid hydrocarbon chain resonances in proton magic angle spinning NMR spectra, we detected the temperature-dependent phase state of naturally occurring lipids of intact influenza virus without exogenous probes. Increasingly, below 41 degrees C ordered and disordered lipid domains coexisted for the viral envelope and extracts thereof. At 22 degrees C much lipid was in a gel phase, the fraction of which reversibly increased with cholesterol depletion. Diffusion measurements and fluorescence microscopy independently confirmed the existence of gel-phase domains. Thus the existence of ordered regions of lipids in biological membranes is now demonstrated. Above the physiological temperatures of influenza infection, the physical properties of viral envelope lipids, regardless of protein content, were indistinguishable from those of the disordered fraction. Viral fusion appears to be uncorrelated to ordered lipid content. Lipid ordering may contribute to viral stability at lower temperatures, which has recently been found to be critical for airborne transmission.