The experiments described in this thesis were designed to investigate the variations in the flow rates of red cell suspensions perfusing an in vitro system of pores, diameters 3 to 200μ, under a variety of experimental conditions. The red cells were suspended in either heparinised plasma or buffered Ringer solution. The flow rates of normal cell suspensions were found to be dependent on perfusion pressure, haematocrit and pore diameter. The flow rates decreasing with decreasing perfusion pressure and increasing haematocrit. The dependence of flow rate upon pore diameter was the inverse of that seen in tubes of diameter 200μ and larger, that is, the flow rate decreased with decreasing pore diameter. The measurements of flow rates were then used as the basis for the calculations of the relative viscosities of the cell suspensions. These values were consistent with the hypothesis that the red cell in flow approximates to a liquid drop and that the suspension as a whole can be described in terms of emulsion theory. True emulsion theory is not however applicable to the smallest pores but a recent theory, based upon liquid drop capillary flow, gave good agreement with the experimental results. The flow rates of red cell suspensions were found to depend also on the pH, temperature and oxygen tension of the suspending medium. The relative viscosities of the suspensions investigated showed a minimum value in the temperature range 35 to 40 C and a similar minimum between pH 7.2 and 7.4. The flow rates were found to be independent of oxygen tension at P(02) above 40 mm Hg but to subsequently decrease with decreasing oxygen tension.The possible mechanisms underlying these observed changes are fully discussed in chapter 5. The effects of cell fixation and incubation were also studied; both factors reduced the flow rates significantly in the case of cell incubation the decreases were related to both the length and temperature of Incubation. Finally a limited number of cell suspensions taken from a series of patients, were studied. It was shown that there were significant decreases in flow rates as compared to the normal in certain groups of patients.