A previous theoretical study on blood flow in capillaries has been extended and improved. The theory neglects the cell membrane's resistance to bending and uses the simplifications of the hydrodynamic theory of lubrication. Cell shape, cell velocity, pressure drop, and flow field are determined from the theory without recourse to empirical parameters. The results are applicable to capillaries of approximately 8 μ diameter and smaller. Detailed comparisons are given with applicable prior experimental and theoretical work on cell shape and pressure drop.