Rheological properties of lysozyme viscoelastic matrices resulting from a sol-gel transition taking place in organic/aqueous media at room temperature were investigated. Gel-like structures, of transparent appearance, developed out of lysozyme (5.0 mmol/dm(3)) dispersed in tetramethylurea (TMU)/water binary mixtures, at TMU mass fraction (w) ranging from w(TMU) 0.6 to 0.9. The wide linear viscoelastic region (LVR) observed, up to strains of 10%, was invariant throughout the TMU concentration range investigated, indicating that the 3D structures of protein matrices, although fragile, are quite flexible and able to withstand great deformation before rupture. Storage (G') and loss (G") moduli continuously increased with increasing TMU concentration, the former at a greater rate, consequently leading systems to a decrease in the loss angle, tandelta. For gels developed out of binary systems at w(TMU)=0.9, creep curves revealed behaviour that very nearly approaches that of a perfect elastic solid. Although gelification under the experimental conditions employed is macroscopically accomplished in a time interval that does not exceed 24 h (for the gel developed out of the solvent mixture of lowest TMU concentration, w(TMU)=0.6), a slight decrease in loss angle can still be detected after that period. Such changes, however, have no effect on the LVR. Relaxation tests indicate that systems comprise at least two dynamically distinct contributions.