Mechanical properties in equilibrium swelling states were examined for two elastomeric polypentapeptides, namely, (GVGVP)(251) and (GVGIP)(260) where G = glycine, V = valine, P = proline, and I = isoleucine. They had been prepared using recombinant DNA technology and cross-linked by gamma -irradiation over the range of doses from 6 to 30 Mrad. Moduli were measured in uniaxial tensile tests. The specific work of fracture was determined as a measure of toughness. Increased doses of gamma -radiation increased the moduli (cross-link density) of hydrogels. The hydrophobic folding and assembly phase transition that occurred on raising the temperature was demonstrated to have significant effects on moduli and fracture properties. Thermoelasticity experiments suggest that the phase transition involves changes in elasticity mechanism. Above the transition temperature, hydrogels were shown to become more viscoelastic.