The fluorescence anisotropy (r) of intracellular NADH was used as a parameter responsive to changes in the intracellular structure and cell integrity of bacteria. When Micrococcus luteus cells were subjected to gentle lysis with lysozyme in the presence of saturating concentrations of respiratory substrates, the respiratory rate was stimulated transiently followed by the decrease in respiratory chain activity correlated with r value reduction. It is concluded that immediately following lysozyme treatment, the respiratory chain in situ then exists in a state characterized by a maximal activity, until such time as cell integrity is disturbed. The preservation of a high respiratory activity in lysed bacterial cells pretreated with glutaraldehyde confirms this.