The mechanical properties of compressed beam specimens of microcrystalline cellulose (Avicel pH 101) have been assessed in terms of the tensile strength (sigma t), Young's modulus (E) and the following fracture mechanics parameters: the critical stress intensity factor (KIC), the critical strain energy release rate (GIC) and the fracture toughness (R). Increase in the compaction pressure used to form the beams resulted in compacts with higher values of tensile strength, Young's modulus, KIC, GIC and R, indicating that the compacts became less brittle as their porosity decreased. Extrapolation of the values of sigma t, E, KIC, GIC and R to provide values at zero porosity indicated that the material had values of 30 Nm m-2, 0.0103 GPa, 1.21 MN m-3/2, 1.98 x 10(2) Nm-1 and 2.19 x 10(3) Nm-1, respectively. These provide a range of values whereby a fuller characterization of the mechanical properties of pharmaceutical materials can be made.