Backbone dynamics of the ligand- (FK506-) bound protein FKBP- 12 (107 amino acids) have been examined using N-15 relaxation data derived from inverse-detected two-dimensional H-1-N-15 NMR spectra. A model free formalism [Lipari & Szabo (1982) J. Am. Chem. Soc. 104, 4546-45591 was used to derive the generalized order parameter (S2), the effective correlation time for internal motions (tau(e)), and the chemical-exchange line width (R(ex)) based on the measured 15N relaxation rate constants (R1, R2) and H-1-N-15 heteronuclear NOEs. The final optimized overall correlation time (tau(m)) was 9.0 ns. The average order parameter (S2) describing the amplitude of motions on the picosecond time scale was found to be 0.88 +/- 0.04, indicating that internal flexibility is restricted along the entire polypeptide chain. In contrast to results obtained for uncomplexed FKBP, the 80's loop (residues 82-87) surrounding the ligand binding site was found to be rigidly fixed, indicating that internal motions at this site are damped significantly due to stabilizing noncovalent interactions with the FK506 molecule. Structural implications of these differences in picosecond mobility as well as possible implications for calcineurin recognition are discussed.