The conformational motilities of three regions of the sperm whale myoglobin molecule and of an isolated peptide of myoglobin have been examined by measuring the equilibrium constant for the native ↔ nonnative transition. The immunological approach of Furie et al. (Furie, B., Schechter, A. N., Sachs, D., and Anfinsen, C. B. (1975), J. Mol. Biol. 92, 497–506) was used with convenient modifications. Antibodies specific to the nonnative conformations were used in assaying for competition between the radioactively labeled peptide and native myoglobin. Labeling was by 125I iodination of the peptide or its 3-(4-hydroxyphenyl)propionyl derivative, and separation of the immune complex from the free peptide was either by ammonium sulfate precipitation or by centrifugation of the antibodies immobilized on Agarose beads. For the antigenic regions of the sequence (1–55), the measured conformational equilibrium constant was 840 ± 200 at 22 °C; the value for the C-terminal region (132–153) was 280 ± 120 at 25 °C, while that for the region (66–76) adjacent to the heme group was greater than 2.5 × 106. Measurements on the isolated peptide (132–153) indicated that 1% of the molecules adopt native-type folding in aqueous solution at 36 °C. © 1977, American Chemical Society. All rights reserved.