A low carbon, microalloyed steel was heat treated to obtain dual phase microstructures containing constant levels of 18 and 25 vol.-% martensite at two levels of microstructural refinement and with varying epitaxial ferrite content. Tensile and compression tests were conducted at a strain sensitivity of 2 x 10(-5). Elastic limits in tension and compression were indistinguishable and very low, suggesting that mobile dislocations were present in the ferrite as a consequence of stress relaxation processes. These mobile dislocations accommodated the volume increase accompanying the austenite to martensite transformation during heat treatment. Epitaxial ferrite had little effect on the 0.2% proof stress, but average proof stresses were generally higher in compression than in tension owing to residual stresses in the martensite and ferrite following heat treatment. The residual stresses calculated from this asymmetry in the proof stresses were small because of stress relaxation in the ferrite at the temperature at which the martensite formed. Epitaxial ferrite significantly increased uniform elongation in tension with a small decrease in tensile strength both levels of martensite in the finer microstructure but only at the 18 vol.-% martensite level in the coarser microstructure. The cause of the increased ductility was the effect of epitaxial ferrite on the work hardening rate between approximately 0.5 and 3% strain; epitaxial ferrite reduced the work hardening rare in this range of strain. MST/4350.