The investigation of non-uniform distribution of erythrocytes across the blood flow are of paramount importance for blood rheology and the physiology of circulation.' A recently developed technique, integral laser Doppler anemometer (IDA), enables the rapid measurement of transverse concentrational profiles (TCP) of particles in suspension flows in narrow channels.2'3 In this technique a channel (or a part of it) has transparent walls, and the integral over channel cross-section Doppler spectra of particles in a flow are registered. For a uniformly illuminated flat channel of width 2h the shape of IDA-spectrum 5(w) is related to the flow velocities profile v (x) and TCP of suspended particles C(x) by Ref. 2,3:Here 1 is the lateral coordinate measured from the central plane, zthe distance along a channel, w -Doppler frequency, qlight-scattering vector andthe broadening of individual Doppler line. For dilute suspension flows, when v(x) = vo(1 -z2/h2), eq. (1) enables a simple calculation of the TCP of particles from the measured 5(w).Such measurements were done for dilute (iO -iO cells/cm3) suspensions of human red blood cells (RBC), flowing ill fiat glass channels of 2h = 30 -300pm at V0 = 1 -10 cm/s. RBCs were freshly drawn from a fingertip and placed in phosphate-buffered saline at p11=7.3. The measurements at various lengths of z showed the displacement of RBCs from the channel walls and their lateral hydrodynamic focusing during advancement along the channel (Fig. 1, x in units of h, v = 8.7 cm/s). Such non-central focusing was originally observed for rigid spheres4 and later for RBCs [1] in circular channels. Our measurements showed that the equilibrium position x of TCP peak depends on the ratio of RBC size to the channel width (Fig. 2, vo = 9 cm/s (o, +), 4 cm/s ( ,x), 2 cm/s ( , ). Furthermore, depending on 2h, two qualitatively different focusing regimes were observed. For 2h < 7Opm RBCs were focused at the central plane of a channel (Fig. 3, vo = 2 cm/s). The change of Xf was rather sharp and was accompanied by the reversal of the dependence of the transient position of TCP peak on the flow velocity (Fig. 4, z =62 mm (o), 32 mm (x), 32 mm (a), 63 mm (+). As Fig. 4 shows, for 2h > 7Opm the TCP peak shifts to the channel center with the increase of v0 , but for 2h < 7Opm the shift is in the opposite direction. The transition from non-central to central-plane focusing occured at (RBC size)/2h ratio s 0.1 and v0 about 1 cm/s. This effect, which was not previously observed, can be explained by the competition of two focusing mechanisms. The first, non-central one, is due to the inertia effects in a fluid, which are quadratic in v0. Thus, it dominates at higher channel Reynolds numbers Re.4 The second seems, in essence, to be analogous to the mechanism of lateral migration of deformable particles to the center of a channel flow,5 but exploits the unique orientational behavior of erythrocytes in a flow. At low shear rates G = (vo/h) RBCs are rotating in a flow, but at G > 's 20s' they maintain a rather stable o...