Stress-strain relationships of bulk of seeds were examined under conditions of uniaxial compression with addi- tional measurement of lateral stress. Tests were performed on wheat, rye, barley, oats and rapeseed for five levels of moisture content in a range from 6 to 20%. A constitutive stress-strain relationship formulated for sand was found to describe adequately the ‘en masse’ mechanical behavior of grain. The elastic constants of the material (Young’s modulus E and Poisson’s ratio n) were determined using the linear phase of the unloading of the sample. Values of the modulus of elasticity E of cereal grains were found in a range from 10.4 to 23.6 MPa and decreasing with an increase in moisture content. Values of E for rapeseed ranged from 9 to 6.6 MPa for moisture contents of 6 and 15%, respectively. Values of Poisson’s ratio n of cereal grains were found in a range from 0.22 (wheat - 10.8% of m.c.) to 0.15 (barley – 15.3% of m.c.), while in the case of rapeseed, values of n ranged from 0.24 for 6% to 0.10 for 15% of m.c.