The aim of biological crystallogenesis is to understand and control crystal growth of proteins. Systematic investigation of the prenucleation stage in the nucleation processes and investigations of the growth kinetics is essential. Quasi Elastic Light Scattering (QELS) and Small Angle X-ray Scattering (SAXS) experiments are carried out in situ to determine interactions between protein molecules and their degree of association, before nucleation occurs. From knowledge of the phase diagram, the early stages of the crystallization process of bovine pancreatic trypsin inhibitor (BPTI) were investigated by QELS, in NaCl and KSCN solutions. With both salts, the solutions became monodispersed, but not monomeric in the vicinity of the solubility curve. In addition, the attractive interactions increased with ionic strength due to the solubility decrease.