A combination of small angle X-ray scattering (SAXS) and gel techniques was used to follow the kinetics of aprotinin (BPTI) nucleation and crystal growth induced in NaCl solutions by increasing temperature. These experiments were carried out in order to determine if aggregation occurs in the solution during crystal growth. From these first series of experiments, we found a radius of gyration of 24.7Angstrom for intermediates species in solution, consistent with a mixture of decamers and monomers. Once formed, BPTI crystals obtained at acidic pH in NaCl kept this association and it is the decamer, which was found in the crystal packing in contrast to what happened at basic pH where only monomers are observed in the crystal. (C) 2002 Elsevier Science B.V. All rights reserved.