The CD3/CD28 Streptamer® approach is based on our Strep-tag® technology and facilitates T cell activation and expansion. Low affinity Fab fragments fused to a Twin-Strep-tag® (Fab-Streps) against CD3 and CD28 are multimerized on a Strep-Tactin® multimer backbone. The resulting CD3/CD28 Streptamer® complex stimulates the T cell receptor/CD3 complex (TCR/CD3; signal 1) and the co-stimulatory molecule CD28 (signal 2). Providing these two signals leads to efficient T cell activation and proliferation. For in vitro stimulation and activation, the soluble CD3/CD28 Streptamers are added to T cells. The addition of biotin reverses the multimerization and causes the dissociation of low affinity Fab fragments from the cells. Consequently, the termination of stimulation is possible at any point in time.