The functional activity of leukocyte in-tegrins is highly regulated by several mechanisms related to intrinsic molecular properties and receptor interaction with the cell membrane. Here, we present a microkinetic study of the lymphocyte function-associated antigen-1-mediated interaction between flowing Jurkat cells and surface-or cell-bound intercellular adhesion molecule-1 (ICAM-1). We conclude that adhesion is initiated by the formation of a single bond with 0.3 s –1 dissocia-tion rate, and attachment is subsequently strengthened by the formation of additional bonds during the next 10 s; exposing cells to Mg 2 or Mn 2 resulted in up to a 16-fold increase of the binding frequency, in line with reported measurements performed on isolated molecules with surface plas-mon resonance methodology; cell-bound ICAM-1 molecules were more efficient in mediating adhesion than Fc-ICAM-1, properly oriented and bound by surface-adsorbed protein A; and quantitative analysis of binding frequency suggested that adhesion efficiency was ten-to 100-fold lower than the maximum value allowed by previously determined association rates of soluble molecules. It is concluded that the presented methodology provides a simple and unique way of dissecting the initial step of cell adhesion and discriminating between affinity and avidity modulation of adhesion receptors.