Plastic deformation under uniaxial longitudinal tension and compression is investigated for pure aluminum reinforced with a high volume fraction of parallel alumina fibers. The matrix sub-structure is also examined in transmission electron microscopy. The aim is to study the in situ room-temperature mechanical behavior, particularly the work-hardening rate, of pure aluminum when reinforced with a high volume fraction of chemically inert ceramic reinforcement. The matrix substructure prior to deformation, composed of cells about 2-mu-m in diameter, is similar to that of highly deformed unreinforced aluminum. Measured compressive composite elastic moduli agree with rule of mixture predictions; however, no elastic regime is found during tensile loading. As tensile deformation proceeds above a strain around 0.05 pct, a constant rate of work hardening is reached, in which the matrix contribution is negligible within experimental error. Upon unloading from tensile straining, Bauschinger yielding begins before the composite reaches zero load, as predicted by the rule of mixtures. The matrix substructure after load reversal retains a 2-mu-m cell size but with greater irregularity in the dislocation configurations. Using the rule of mixtures, in situ stress-strain curves are derived for the reinforced aluminum matrix and described by a modified Voce law.