The structure and stability in solution of the monomeric form of GroEL were studied by the methods of circular dichroism, binding of a hydrophobic probe, limited proteolysis, modification of thiol groups, sedimentation, and size-exclusion chromatography. The monomeric GroEL at 23 degrees C was shown to be a globular protein with a pronounced secondary and a rigid tertiary structure. It exhibited no marked tendency to oligomerization in the absence of adenine nucleotides. However, the free monomeric GroEL was substantially less stable to urea and heat than the corresponding subunit in the composition of native oligomeric particles. The monomeric form also bound the hydrophobic probe, 8-anilino-1-naphthalenesulfonic acid, by an order of magnitude better than the subunit in the oligomeric particles. The ATP-induced oligomerization process of both folded and unfolded GroEL monomers was studied. The oligomerization rate was found to be the same for both monomers, and, therefore, should be limited by the ATP-dependent "arrangement" of the sites in the folded monomers responsible for the oligomerization rather than by the spontaneous refolding of monomers.