The behaviour of canine erythrocytes in isotonic saline suspensions undergoing steady unidirectional shear flow in the vertical direction, at cell volume concentrations (ρ) ranging between 0.017 and 0.840, is studied by measuring suspension resistivities in three mutually orthogonal directions. Results are compared with the predicted behaviour of rigid spheroids and liquid drops. Analyses of suspension resistivities show that there are significant changes in erythrocyte shape with increasing shear rate (
k
) at all
k
covered (2.6 x 10 – 10
3
s
-1
) and at all ρ, very large changes occurring at ρ > 0.65. However, the changes in the equivalent principal axis-ratios of erythrocytes with
k
deviate substantially from the flow behaviour expected of liquid drops, the average ‘shape factor’ for the cells decreasing rather than increasing with
k
. Neither the simple liquid drop nor the rigid spheroid model can adequately explain some aspects of the results obtained. Results show that cell concentration is a major determinant of erythrocyte behaviour and point to the existence of three concentration regimes in the range of flow rates investigated, with different mechanisms of particle orientation during shear flow predominating in each. At ρ < 0.08 no significant change in orientation distribution with
k
is observed; but results show that erythrocytes tend to assume the orientation corresponding to the maximum dissipation of energy in a shear field or corresponding to the maximum energy dissipation of settling disks (the effect increasing as ρ is reduced) indicating a predominance of inertial effects. Between ρ ≃ 0.08 and ρ ≃ 0.65 the orientation distribution changes towards alignment of erythrocytes parallel to the plane of flow, the degree of alignment increasing with
k
and with ρ. Here, particle-particle interactions facilitate the orientation of erythrocytes by shear stresses. Above ρ ≃ 0.65 the change in cell shape with
k
increases sharply, suggesting that erythrocyte deformation becomes the principal mechanism of suspension flow.
Particle orientation distribution in γ-Fe<inf>2</inf>O<inf>3</inf>magnetic tapes by Mössbauer and hysteresis loop measurements