Small-angle neutron scattering experiments are often performed with proteins solubilized in heavy water because of the large difference in neutron scattering properties of protons and deuterons. In order to characterize the effect of D2O on physico-chemical properties of protein solutions, we investigated the effect of D2O on the phase diagram and the interactions of bovine pancreatic trypsin inhibitor (BPTI) in solution. We measured the solubility in D2O of BPTI solutions in the presence of NaCl (reverse solubility) and KSCN (direct solubility) and compared with the values measured by Lafont et al. in H2O under the same conditions [Lafont et al., J. Crystal Growth 173 (1997) 132]. In the two salts, we found that BPTI solubility in D2O is significantly lower than in H2O. The curves representing the solubility of BPTI in KSCN are shifted by 7.2 degrees C between light and heavy water, a shift obtained previously with lysozyme and representing the difference in the temperature of maximum density of both types of water [Gripon et al., J. Crystal Growth 177 (1997) 238; 178 (1997) 575]. In the case of BPTI in NaCl, we did not find this relationship between the solubility in H2O and D2O. We found, by dynamic light scattering, that BPTI attractive intermolecular interactions in the presence of NaCl in D2O are significantly stronger than in H2O. We investigated the association of BPTI molecules in crystallization conditions in the presence of NaCl in H2O and D2O by small-angle X-ray and neutrons scattering, respectively. In the presence of heavy water, the transition monomer-multimer is observed at about 2 mg/ml of BPTI in 1 M NaCl whereas in light water and in 1.4 M NaCl solution this transition is observed at about 15 mg/ml. These results dearly showed that BPTI in crystallization conditions is a multimer and confirm the importance of the isotopic nature of water in the crystallization of proteins. The replacement of H2O by D2O decreases the solubility and increases the attractive intermolecular interactions.