A theoretical analysis is presented to describe and compare two unit operations that separate molecules on the basis of size : ultrafiltration and size-exclusion chromatography. A model consisting of a series of N ultrafiltration units is set up to describe the fractionation of macrosolutes fed to the system in the form of a square pulse. This model is found to be equivalent to the plate model used to describe elution profiles in linear chromatography. Likewise, for a series with large number of units, the solutes elute from the system in the form of a Gaussian distribution. The analogy between this multistage ultrafiltration and column chromatography is further extended by defining and comparing parameters usually used to describe chromatographic separations such as resolution, capacity and selectivity. A case study is presented that compares the theoretical behaviour (elution profiles, resolution) of multistage ultrafiltration with size-exclusion chromatography, when fractionating a mixture of proteins. As expected, the multistage ultrafiltration behaves in an opposite fashion when compared with size exclusion chromatography, by eluting solutes in increasing order of their size. Furthermore, it is shown that multistage ultrafiltration is more efficient than size-exclusion chromatography, since similar resolution can be obtained but with a lower number of stages. It is anticipated that stacks of ultrafiltration membranes could provide a new type of chromatography : ’reverse’ size-exclusion membrane chromatography. It is also suggested that stacks of UF membranes with large pores to overcome hydrodynamic drawbacks and smaller diffusive pores - ’perfusion’ membranes - can perform ’normal’ size-exclusion membrane chromatography.