We have recently reported that actin modified with dimethyl suberimidate takes a filamentous form even under depolymerizing conditions, and this phenomenon is accounted for by the conformational fixation caused by the introduction of an intramolecular cross-link (Ohara, O., Takahashi, S., Ooi, T., & Fujiyoshi, Y. (1982) J. Biochem. 91, 1999-2012). The suberimidate-treated actin (SA) is not immediately depolymerized by deoxyribonuclease I (DNase I) but is depolymerized after incubation for one day, i.e., depolymerization is much slower than that for intact F-actin. The results on circular dichroic spectra of a mixture of SA and DNase I suggest that DNase I flips the conformation of SA into a G-actin-like state from the F-actin-like one when a tight SA-DNase I complex is formed. The suberimidate cross-link introduced in an SA molecule does not completely prevent the conformational change from the F-state to the G-state but stabilizes the actin conformation very greatly in the F-state.