Byssal threads provide marine mussels with the tenacity to remain sessile in habitats of high flow. Under uniaxial tension, byssal threads are typical of other biological and synthetic fibers in exhibiting an initial linear region followed by yield. They differ, however, in their capacity to recover or "self-heal" following yield. We have examined the effect of urea, dithiothreitol (DTT), and metal-chelating agents such as ethylenediamine tetraacetic acid (EDTA) in perturbing the modulus, yield point, and energy dissipated in distal byssal threads stretched cyclically in seawater to a strain of 0.7. Threads stretched in the presence of 8 M urea or DTT show a complete abolition of yield point, while those washed clean of urea and DTT prior to stretching approach native controls. Threads stretched in the presence of EDTA show no effect; however, preincubation of distal threads in EDTA for 24 h results in a loss of yield point if stretched in metal-deficient seawater and normal behavior in natural seawater. The results indicate that while protein unfolding and disruption of disulfide linkages or chelate complexes compromises the yield strength of distal byssal fibers, there is typically a rapid recovery in natural sea water.