Hyperpolarization in human leukemia THP-1 monocytes adherent to vascular cell adhesion molecule (VCAM)-1 is due to an induction of inwardly rectifying K⁺ currents (I_ir) (Colden-Stanfield M and Gallin EK, Am J Physiol Cell Physiol 275: C267-C277, 1998). We determined whether the VCAM-1-induced hyperpolarization is sufficient to augment the increase in intracellular free calcium ([Ca²⁺]_i) produced by Ca²⁺ store depletion with thapsigargin (TG) and readdition of external CaCl₂ in fura 2-loaded THP-1 monocytes. Whereas there was a 2.1-fold increase in [Ca²⁺]_i in monocytes bound to glass for 5 h in response to TG and CaCl₂ addition, adherence to VCAM-1 produced a 5-fold increase in [Ca²⁺]_i. Depolarization of monocytes adherent to VCAM-1 by I_ir blockade or exposure to high [K⁺] abolished the enhancement of the peak [Ca²⁺]_i response. In monocytes bound to glass, hyperpolarization of the membrane potential with valinomycin, a K⁺ ionophore, to the level of hyperpolarization seen in cells adherent to VCAM-1 produced similar changes in peak [Ca²⁺]_i. Adherence of monocytes to E-selectin produced a similar peak [Ca²⁺]_i to cells bound to glass. Thus monocyte adherence to the physiological substrate VCAM-1 produces a hyperpolarization that is sufficient to enhance Ca²⁺ entry and may impact Ca²⁺-dependent monocyte function.