The reciprocal sound velocity of an ideal solution is, according to the model of Natta and Baccaredda, composed of the reciprocal sound velocities of its components weighed by their volume fractions. This enables us to extract the apparent sound velocity of solutes, and we applied this procedure to a lysozyme solution. With ultrasonic velocimetry, we have obtained an apparent sound velocity of 1958 m/s which is in good agreement with values determined by the ultrasonic pulse-echo method for hydrated lysozyme crystals. (C) 2002 Elsevier Science B.V. All rights reserved.