Transforming growth factors beta (TGF-beta) are cytokines with multiple biological activities. Their development as biopharmaceutical drugs targets the control of complex physiological processes such as osteogenesis and epithelial cell differentiation. We report here the first characterization of the recombinant human (rh) TGF-beta 2 and rhTGF-beta 3 isoforms in terms of their conformational stability and structural transitions induced by a chaotrope or temperature. The transitions detected by CD spectroscopy suggested that thermal denaturation of both TGF-beta isoforms apparently fitted a simple two-state (N<==>D) model. However, the ratios of calorimetric to van't Hoff enthalpies, significantly different from unity, indicated that these molecules most probably consist of independently denaturing subdomains. The complex transitions induced by guanidine hydrochloride, at pH 1.8 or 8.0, also suggested intermediately denatured structures. Thermodynamic stabilities under pH conditions useful for bioprocessing were derived from spectroscopic and calorimetric measurements. Treatment of thermal denaturation data by van't Hoff analysis yielded, for the beta 2 and beta 3 isoforms respectively, apparent delta G(25 degrees C, pH 1.8) of 20.4/17.2 kJ/mol and 17.5/18.6 kJ/mol (near-UV CD/far-UV CD data) in 20 mM hydrochloric acid, and apparent delta G (25 degrees C, pH 3.0) of 35.1 and 33.5 kJ/mol in 0.25 M acetic acid (calorimetric data). Neither low-pH-induced denatured states nor soluble aggregates were detected in both acidic solvents. The spectroscopic and thermodynamic data should be useful for assessing the homogeneity and proper folding of these recombinant molecules.