Early events in the partitioning process which involve characteristic kinetics of cell- and phase-specific interactions and phase separation have been described previously. This paper reports on red cell—phase droplet interactions pertaining at the time of usual phase sampling ( i.e., the time at which a clear bulk interface is first apparent) and beyond in cell partitioning and countercurrent distribution experiments. In non-charge-sensitive phase systems close to the critical point, cells can be free or attached to phase droplets. Cells that are free are virtually completely in the top phase, whereas different cell populations that show essentially complete binding to droplets can nevertheless have different partition ratios and be separated, thus reflecting the effects of the difference in the cells' avidity for the phase droplets during the early, elapsed events in partitioning. At higher polymer concentrations ( i.e., higher interfacial tensions), the cell populations, completely bound to phase droplets, partition completely to the interface, and consequently cannot be separated. When such systems are made charge-sensitive by the generation of a Donnan potential between the phases or made into affinity systems by the incorporation of PEG ligands ( e.g., PEG—palmitate), there is a decrease in the avidity of the cells for phase droplets. The resulting increase in the ratio of free to droplet-bound red cells in the top phase at the time of sampling correlates with an increase in the partition ratio, P, observed. The results show that a major difference (at the time of sampling) between non-charge-sensitive and charge-sensitive or affinity systems is that only in the former can cell populations be separated when they are almost completely bound to droplets. It was also found that although red cell—droplet interactions still pertain at the usual sampling times, there is a reduced involvement of kinetics at this late phase separation stage. This is operationally important as it provides leeway in selecting the sampling (settling) time.