Abstract The limits of validity of the semiclassical theory in which gravity is unquantized are discussed. This is done by comparing the emission of classical gravitational waves in the semiclassical theory with graviton emission in quantum gravity theory. It is shown that these can be quite different even for macroscopic systems. Thus quantum gravitational effects can manifest themselves on a macroscopic scale. A hypothetical experiment to demonstrate the existence of gravitons by means of such effects is discussed.