The design of a separation process for mixtures of ampicillin and phenylglycine crystals by interfacial partitioning is treated. Interfacial partitioning is a novel technique where particles can be separated in a liquid two-phase system by selective partitioning between the liquid phases and a third particle-rich layer positioned at the interface. There are different mechanisms of separation with distinguishable equilibrium partition relations. Several short-cut design procedures for multistage counter-current fractionation of binary particle mixtures are presented. The design procedures relate to the different equilibrium partition relations and deal with the issue of flow selection. One of the procedures is applied for the design of a separation process for mixtures of phenylglycine and ampicillin crystals in water/n-dodecane two-phase systems. It is experimentally shown that multistage operation of the process can lead to a better separation than a single stage partition process. Besides, it is shown that the design can be complicated by in-eversible effects in the partition behavior of the crystals. Additionally, results are presented of experiments with a semicontinuosly operated partition unit, which indicate the possibility of a fully continuous operation of interfacial partitioning. A considerable degree of separation can be achieved provided that thorough contact between both liquid phases and the particles is maintained.