The behavior of a high molecular weight(48.8 x 10(3)) synthetic random copolymer composed of Glu, Leu, and Trp residues in molar ratio 82.5:16:1.5 under compaction and unfolding conditions has been studied using circular dichroism, fluorescence spectroscopy, viscometry, and size-exclusion chromatography (FPLC). It has been shown that the decrease of the pH value leads to two-stage compaction of the copolymer molecules. This means the formation of an intermediate conformational state. Structural properties and stability of the revealed conformations (compact state and less compact intermediate) of the copolymer toward temperature and urea-induced unfolding have also been investigated. The compact conformation of the macromolecules is characterized by high resistance toward urea action (at 21.5 degrees C), whereas the transition from intermediate (less compact) state to coil-like conformation can easily be accomplished by the addition of small amounts of urea (at 21.5 degrees C). Melting of the compact state in the presence of urea is a two-stage process as well. This fact also confirms the existence of a folding intermediate.