1. Osmotic haemolysis of a standard suspension of human red cells was followed using a recording spectrophotometer at wave-lengths between 600 and 650 mmu.2. Optical density changes in the cell suspensions were related to turbulence of the suspension, cell swelling and loss of haemoglobin-containing cells.3. The time course of the loss of cells from suspension in hypotonic electrolyte solutions showed two phases: an early rapid phase was followed by a smaller phase of longer half-time.4. The second phase was most prominent in the middle ranges of partial haemolysis and less when total haemolysis was at the extremes of the detectable range.5. The second phase was eliminated by the inclusion of 20 mM sucrose in the suspension, and was slowed by the presence of 0.005% tannic acid without alteration of magnitude.6. The magnitude of the second phase was dependent on the dominant external cation, becoming progressively greater through the series: Mg(2+) < Na(+) < Li(+) < K(+) < Rb(+).7. The slow phase is interpreted as arising from passive cation permeability in cells swollen to a volume close to that critical for haemolysis, with water influx secondary to the unopposed colloid osmotic pressure of intracellular protein.