Cowpea chlorotic mottle virus (CCMV), which is stable at pH 5.0, has been modified at this pH with 0.5--0.7 pyridoxal 5'-phosphate molecules per protein subunit. The fluorescence properties of the labelled CCMV protein in different aggregation states of the virus provide information about the labelled part of the protein and the changes induced in its environment, when the nucleo-protein particles are swollen or dissociated. Fluorescence excitation and emission spectra indicate the presence of radiationless energy transfer from the aromatic amino acid residues to the label. Comparison of the fluorescence lifetimes of the labelled and the unlabelled protein confirms the existence of energy transfer. The mobility of the labelled part, which can be estimated from the fluorescence polarization of pyridoxal phosphate chromophore, is higher than expected from the dimensions of the virus and the protein subunits. Polarization values and the fluorescence lifetimes depend on the presence of small amounts of NaCl or MgCl2 in the buffer solution at pH 7.5. This is due to structural changes in the vicinity of the pyridoxal phosphate label of the RNA and of the protein part.