The influence of microplastic deformation on the strength characteristics of the eutectic Sn–38% wt. Pb alloy is studied. The mechanical tests of alloy samples are carried out at the stretching up to relative elongations not exceeding 3$\cdot10^3$. The deformation of the samples is determined using strain gauge method. The experiments are carried out in the mode of active loading at room temperature at which the alloy shows superplastic properties. The alloy is studied in three different states, which differ in duration of natural aging. The obtained dependences of the stress on the value of the relative elongation of samples indicate a significant strain-softening of the studied alloy in contrast to the strengthening of metal materials usually observed under similar conditions of mechanical testing. In all cases, after unloading, significant compression of the samples is observed over a period of time, which indicates the presence of significant internal stresses in the material. The reasons for strain-softening are discussed taking into account previously obtained data on changes in internal friction and Young’s modulus of the alloy after cyclic deformation in the microplasticity range in the mode of resonant bending oscillations. As concluded, the alloy strain-softening is caused by the relaxation of internal stresses and structural-phase transitions under conditions of external loading of samples. The observed changes of strain-hardening coefficient can indicate the metastable phase state of the alloy and realization of initial stages of decay of supersaturated solid solutions, stimulated by external mechanical tensile stress, in all investigated cases.