Recombinant porcine growth hormone (rPGH) was solubilised from inclusion bodies (lB's) using either 6 M guanidinium hydroehloride (GnHCI). 7.5 M urea or by a novel method using a cationic surfactant, cetyltrimeth)'lal~monium chloride (CTAC). Circular dichroism (CD) analysis of the secondary (2 °) structure of the urea-and GnHCl-sol ubilised rPG H showed the absence of,r-helical content with the majority of the molecule existing in a "random coil" structure. In contrast, the CTAC-solabilised rPGH displayed significant starting 2 ° structure (10-15% ~ helix; 30-40% fl structure). The three rPGH preparations were refolded in vitro against weak urea, GnHCI or aqueous buffers, resulting in an average refolding ¢ffieiel~cy of 50% native (monomeric) rPGH for CTAC solubilised IB's and only 20% for urea or GnHCI solubilised IB's. We conclude that the method of solubilisation of IB's and the resultant difference in the starting 2 ° structure of rPGH, particularly ,r.helical content, is a major in vitro factor that apparently predetermines the aggregation/refolding behaviour rPGH irrespective of refolding environment.