The viscoelastic properties of leukocytes (WBCs) during small deformation were determined by micropipette aspiration. The passive deformation behavior of neutrophils suspended in a Ca2+-free medium in response to a step aspiration pressure consists of an initial rapid, elastic response followed by a creep displacement. These time-dependent responses can be modeled by a viscoelastic solid in which an elastic element (K1) is in parallel with a Maxwell element composed of another elastic element (K2) in series with a viscous element (mu). Variations in temperature (9-40 degrees C) cause an inverse change in mu, but have no effects on K1 and K2. All three coefficients are not affected by decreases in pH down to 5.4; with pH greater than or equal to 7.8, however, K1 and mu increase. Increases in osmolality cause a rise in all three coefficients, especially mu. Colchicine treatment results in selective decreases in mu and K2 without affecting K1. B lymphocytes have viscoelastic coefficients similar to those of neutrophils, but T lymphocytes have higher values for these coefficients. In the presence of 2 mM Ca2+, the neutrophils have higher viscoelastic coefficients than in Ca2+-free medium, and they form protopods which have greater resistance than the main cell body to deformation by micropipette aspiration. Morphometric analysis shows that WBCs have large excess membrane area due to the presence of membrane foldings, which facilitate WBC deformation at constant area. During filtration through 5-micrometers sieves, WBCs are much more prone to pore plugging than erythrocytes because of their higher cellular viscosity and the presence of nucleus. The rheological properties of WBCs have significant implications in their functions and flow dynamics in the microcirculation.