As the spectra and binding parameters calculated for the thiouracil-albumin interaction change with the protein concentration, a human seroalbumin conformational change depending on protein concentration has been suggested. This protein-conformational change is tested by dilatometry and viscosimetry. At low concentrations, albumin showed a greater thiouracil binding capacity and a second positive peak in its interaction with the drug, detected by difference spectroscopy. Both effects are due to a monomerisation of protein dimers and not to a conformational change depending on protein concentration. This monomerisation would imply a major accessibility of thiouracil and propylthiouracil to other binding sites on HSA.