High-throughput label-free measurements of the optical and mechanical properties of single microparticles play an important role in biological research, drug development, and related large population assays. However, mechanical detection techniques that rely on the density contrast of a particle with respect to its environment cannot sense neutrally bouyant particles. On the other hand, neutrally buoyant particles may still have a high compressibility contrast with respect to their environment, opening a new window to their detection and analysis. Here we present a label-free high-throughput approach for measuring the compressibility (bulk modulus) of freely flowing microparticles by means of resonant measurements in an opto-mechano-fluidic resonator.