Helical content (fα) of bovine mercaptalbumin (BMA) showed the characteristic two‐step decrease in the acidic region, one corresponding to the N→F transition (pH 4.40→3.75; fα, 0.68→0.58) and the other to the F→E transition (the acid‐expansion) (pH 3.60→2.90; fα, 0.58→0.48). However, fα of human serum albumin (HSA) mainly decreased in the N→F transition (N→F, pH 4.6→3.4; fα, 0.70→0.55 and F→E, below pH 3.0; fα, 0.55→0.52). The difference in pH‐profile of fα between BMA and HSA might be due to the microheterogeneity. The 1H‐NMR spectra and cross‐relaxation times (TIS) from irradiated to observed protein protons, which reflect the structural fluctuation and/or mobilability in proteins, were measured on the N‐, F‐, E‐forms of HSA and BMA, and the N*‐form (8.23 M urea, neutral pD) of iodoacetamide‐blocked HSA (IA‐HSA) and bovine serum albumin (IA‐BSA). The 1H‐NMR spectra and elongations of TIS values for the F‐ and E‐forms of HSA and the E‐form of BMA were quite similar to those for the N*‐form of IA‐HSA and IA‐BSA, indicating the liberation of the intramolecular motion in the F‐ and E‐forms. Those for the F‐form of BMA were intermediate between the N‐ and E‐form. The present results together with the reported data on hydrodynamic radii and D–H exchange reaction, indicate that the F‐form of HSA and presumably BMA has a native‐like globule form with a highly helical state and fluctuating tertiary structure. Thus, all of the present findings on the F‐form of serum albumin seem to be in accord with the structural features for the F‐form suggested by Foster's group (1‐3, 19, 20, 22, 23) and the molten globule state demonstrated by Dolgikh et al. (40), and Ohgushi and Wada (36, 37).