The effect of radiation force on a drop of liquid in an acoustic field is examined. It is established that the force depends on the ratio of the densities of the liquid and the drop and on their adiabatic elastic moduli Keywords: acoustic wave, plane wave, sound pressure, liquid sphere, radiation force Introduction. Various issues of motion and interaction of liquid drops, gas bubbles, and cavities were addressed in [2, 3, 8, 13, etc.]. The motion of drops in an acoustic field under radiation forces is of certain technological interest. A radiation force acting on an obstacle in an acoustic field comes about from a change, within some volume, in the time-average momentum carried by a wave and is determined by the integral of the time-average sound (radiation) pressure over the surface of the obstacle. Note that the exact values of radiation pressure in Lagrangian and Eulerian coordinate systems are generally different [1]. In the former case, it is defined as the time-average sound pressure on the surface oscillating in an acoustic field. In the latter case, it is defined as a convolution of the momentum flux density tensor and the normal unit vector to the surface.To determine the radiation force (independent of time) acting on an obstacle in an acoustic field, the liquid pressure is calculated up to the second-order terms, which are due to the nonlinearity of the acoustic field and, hence, do not vanish after averaging over time. The obstacle changes the acoustic field by generating a reflected wave. Therefore, it is necessary to solve a diffraction problem. If the obstacle is a gas bubble, the oscillatory processes excited by the incident wave in the bubble may affect the wave scattering significantly: if the frequency of the wave and the natural frequency of the bubble are close, then the effective scattering diameter will be many times that of the bubble [4,7]. As a result, the reflected wave noticeably contributes to the radiation force.The radiation force can be determined in several steps. The first step is to identify the wave scattered by the obstacle. Since the liquid pressure can be determined up to second-order terms in linear approximation [10], the reflected wave can be identified by solving a linear diffraction problem. The solution of this problem is used at the second step to determine the resultant force exerted by the liquid on the obstacle. At the third step, this force is averaged over time to filter out the constant (radiation) force and to study the behavior of the obstacle under the radiation force.Since the wave reflected from the obstacle is determined by solving the linear diffraction problem, the interaction of the incident and reflected waves is neglected. In what follows, we will use the above approach to examine the case where the obstacle in a liquid is a drop of liquid of other kind. The cases of single solid particles and systems of such particles were discussed in [9].1. Diffraction Problem. Let us first identify the wave reflected from a liquid drop. If the drop is a sphere o...