Rabbit skeletal muscle G-actin on heating is transformed into the G1-state in which the intrinsic fluorescence spectrum is shifted to a longer wavelength compared with that of native actin, but of much shorter wavelength than that of actin in 8 M urea. A structure with fluorescence characteristics identical to those of the G1-form appears upon the removal of Ca2+, upon partial denaturation in 3-5 M urea and renaturation from the completely unfolded form in 8 M urea as well as spontaneously during storage of actin solutions. All this allows us to regard the G1-form of the actin macromolecule as an "intermediate" state. However, in contrast to other proteins in the intermediate state, a band of the CD spectrum has been observed for G1-actin, with an amplitude comparable to that of native proteins in the region where aromatic groups absorb. This points to a relatively low level of intramolecular mobility of the side chains in this structural state of actin. Moreover, according to polarized fluorescence measurements, the G-G1 transition is accompanied not by an increase - as would have been expected, but by a decrease in mobility of the tryptophan residues. The data obtained confirm the previously observed regularity of the intramolecular mobility of tryptophan residues in a hydrophobic environment being often greater than that of tryptophan residues whose microenvironment is formed by polar protein groups.