To investigate the influence of rigid, second-phase particles on the strength of marble in the semibrittle deformation regime, we performed conventional triaxial mechanical tests on synthetic samples formed by hot isostatic pressing two-phase aggregates of fine-grained calcite and 5--20 wt % of either SiC, Al2O3, or SiO2. The samples were tested at strain rates of 10-5 s-1 and at room temperature over a range of pressures from 5 to 300 MPa. Microstructures were studied using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Adding rigid inclusions with incoherent matrix/particle interfaces increased the range of pressure over which the transition between localized brittle failure and crystal plastic flow occurred. The strength of the pure marbles was inversely proportional to porosity between 0 and 8%. In the brittle field, two-phase aggregates are generally 50 MPa weaker than pure marbles of similar porosity. Second phases and associated cracklike porosity tend to distribute brittle deformation in our samples. Aggregates with 20% inclusions show stable cataclastic flow at 5 MPa confining pressure. At the same pressure, pure samples fail catastrophically along a distinct fault. At higher pressure in the semibrittle field, the strengths of pure and two-phase aggregates converge. The deformation resistance of the material containing particles is consistently higher than that of the pure samples. TEM observations also suggest an additional contribution to the deformation resistance and hardening rate by interactions between the dispersed particles and crystal defects. ¿ American Geophysical Union 1993