The deformability of brittle composites, including a fiber-reinforced material, and the effect of plastic deformation on the tensile strength of such materials is investigated. The materials of interest are high-strength steel, carbide, and a composite of aluminum reinforced by 10 vol percent of Al3Ni fibers of 0.7 to 0.8 μ diameter. The Al-Ni composite was produced by unidirectional solidification. The plastic deformation was performed by hydrostatic extrusion into a receiver pressure. The effect of environmental pressure in inducing sound flow in otherwise brittle material is presented. When 50 percent reduction in area in the fibrous material was introduced by extruding into a receiver pressure of at least 150,000 psi, a sound product resulted. The micrographs indicate that the voids formed by fracture of the fibers were healed by flow of the aluminum matrix. For evaluation of the tensile test results, a strength equation for fibrous materials was introduced. This equation, developed by the upper bound approach, predicts both the strength of the composite and whether fibers will fracture. Both the strength and the fracture criterion are functions of volume ratio, strength ratio of the two constituents, geometry, and environmental pressure. The upper bound equation is general and includes the “rule of mixtures” as a special case. Correlation was found between predicted and measured strength for the billets tested. In general, the fibrous material after extrusion is more ductile than “as solidified” but has a lower strength. Shorter fibers are expected to yield lower strength and increased ductility.