It is possible, within the framework of general relativity, to define an active gravitational mass density of incoherent matter. It is not equal to the inertial mass density, except when at rest. The concept can be specialized to a single massive particle; again, its active gravitational mass is not equal to its inertial mass, except when it rests. A measurement of the impulse imparted to a test particle by a massive body passing nearby can establish the difference, and it may be possible to carry out this measurement in a laboratory. As a by-product of our computations we obtain a generalization to nonradial motion of the slowing-down effect in a Schwarzschild field.