We have analyzed the relationship between P-wave velocities, measured at pressures of 40, 100, and 200 MPa, and modal mineralogy in oceanic gabbro samples from ODP Holes 735B, 894G, and 923A. At all pressures, velocities increase with increasing pyroxene content and decreasing alteration (phyllosilicate and amphibole content), but not with increasing olivine content. A Voigt-Reuss-Hill inverse model reveals that the effective bulk densities and elastic moduli of the plagioclase, pyroxene and amphibole phases are near their expected values, but the effective bulk density and elastic moduli of olivine in the gabbros are anomalously low, probably because the olivine is iron rich, and/or because the olivine grains contain ubiquitous networks of cracks. On average, gabbros with velocities typical of seismic layer 3 (6.7 to 7.0 km s-1) contain 5-15% alteration products, including 5-15% amphibole and 0-5% phyllosilicates. These results suggest that the lower crust is slightly to moderately altered. However, total alteration of the lower oceanic crust could be as low as 2%, if the lower crustal gabbros contain abundant olivine grains that have low densities and elastic moduli.