During T cell activation, the engagement of costimulatory molecules is often crucial to the development of an effective immune response, but the mechanism by which this is achieved is not known. Here, it is shown that beads attached to the surface of a T cell translocate toward the interface shortly after the start of T cell activation. This movement appears to depend on myosin motor proteins and requires the engagement of the major costimulatory receptor pairs, B7-CD28 and ICAM-1–LFA-1. This suggests that the engagement of costimulatory receptors triggers an active accumulation of molecules at the interface of the T cell and the antigen-presenting cell, which then increases the overall amplitude and duration of T cell signaling.