Abstract: A method of screening compounds for their ability of inhibiting ligand-induced co-stimulatory receptor internalisation pathways in immune competent human cells is described. The immune competent human cells are incubated at conditions capable of inducing co-stimulatory receptor internalisation in the presence of at least one test compound and the suppression of the ligand-induced co-stimulatory receptor internalisation is determined. There is also described a kit for use in such a method, as well as an immunoregulatory drug capable of blocking down-modulation of a ligand-induced receptor.