Guanidinium chloride- (GdmCl-) induced unfolding of beta-lactamase has been investigated by a combination of size-exclusion chromatography (SEC-FPLC) and usual optical methods. It has been shown that at low temperatures this protein unfolds through two equilibrium intermediates. The first of these intermediates is the molten globule state, while the other (which we have called a "partly folded" state) is less compact than the molten globule but much more compact than the unfolded state. It also preserves a substantial part of secondary structure of the native or molten globule state. We suggest that this new "partly folded" state of a protein molecule can be the equilibrium counterpart of the first kinetic intermediate of protein folding, formed within a few milliseconds, i.e., after the "burst" stage of folding.