The recoilless fraction $f$ in monoatomic microcrystals is calculated from a lattice-dynamical model. Its dependence on the physical boundary conditions at the surfaces of the crystallites is shown to follow the rule that $f$ increases with increasing stiffness of the binding to the surrounding medium. A modified form of the Debye approximation, which has been used in the past to explain M\"ossbauer-effect experiments on microcrystals, is found to yield results which are incompatible with those derived from lattice dynamics. The reasons for the failure of the Debye approximation to predict the recoilless fraction in microcrystals are discussed.