An in vitro method of reversibly staining a target cell, said target cell comprising a receptor molecule on the surface thereof, with a detectable label, the method comprising: contacting a mixture of cells comprising said target cell with (i) a receptor binding reagent, the receptor binding reagent comprising at least one binding site B, wherein the binding site B specifically binds to said receptor molecule, wherein the dissociation rate constant (k off) as determined by surface plasmon resonance for the binding between said receptor binding reagent via the binding site B and said receptor molecule has a value of 0.5 x 10 -4 sec -1 or greater, the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C is able of being reversibly bound to a binding site Z of a multimerization reagent, (ii) a multimerization reagent, the multimerization reagent comprising at least two binding sites Z for the reversible binding of the binding partner C of the receptor binding reagent, wherein the receptor binding reagent (i) and the multimerization reagent (ii) form multivalent binding complexes that bind to said target cell, each multivalent binding complex comprising at least two of said receptor binding reagents bound to one said multimerization reagent, said multivalent binding complex providing increased avidity, relative to said receptor binding reagent; and (iii) said detectable label bound or capable of binding to said multivalent binding complex, wherein said target cell is stained by binding of said multivalent binding complex to said target cell, and wherein staining of said target cell is reversible upon disruption of the binding between said binding partner C of said receptor binding reagent and said binding sites Z of said multimerization reagent, wherein said receptor binding reagent which specifically binds to said receptor molecule is selected from the group consisting of an antibody, an divalent antibody fragment, a monovalent antibody fragment, a proteinaceous binding molecule with antibody-like binding properties and an MHC molecule, wherein said target cell is a mammalian cell. contacting a mixture of cells comprising said target cell with (i) a receptor binding reagent, the receptor binding reagent comprising at least one binding site B, wherein the binding site B specifically binds to said receptor molecule, wherein the dissociation rate constant (k off) as determined by surface plasmon resonance for the binding between said receptor binding reagent via the binding site B and said receptor molecule has a value of 0.5 x 10 -4 sec -1 or greater, the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C is able of being reversibly bound to a binding site Z of a multimerization reagent, (ii) a multimerization reagent, the multimerization reagent comprising at least two binding sites Z for the reversible binding of the binding partner C of the receptor binding reagent, wherein the receptor binding reagent (i) and the multimerization reagent (ii) form multivalent binding complexes that bind to said target cell, each multivalent binding complex comprising at least two of said receptor binding reagents bound to one said multimerization reagent, said multivalent binding complex providing increased avidity, relative to said receptor binding reagent; and (iii) said detectable label bound or capable of binding to said multivalent binding complex, wherein said target cell is stained by binding of said multivalent binding complex to said target cell, and wherein staining of said target cell is reversible upon disruption of the binding between said binding partner C of said receptor binding reagent and said binding sites Z of said multimerization reagent, wherein said receptor binding reagent which specifically binds to said receptor molecule is selected from the group consisting of an antibody, an divalent antibody fragment, a monovalent antibody fragment, a proteinaceous binding molecule with antibody-like binding properties and an MHC molecule, (i) a receptor binding reagent, the receptor binding reagent comprising at least one binding site B, wherein the binding site B specifically binds to said receptor molecule, wherein the dissociation rate constant (k off) as determined by surface plasmon resonance for the binding between said receptor binding reagent via the binding site B and said receptor molecule has a value of 0.5 x 10 -4 sec -1 or greater, the receptor binding reagent further comprising at least one binding partner C, wherein the binding partner C is able of being reversibly bound to a binding site Z of a multimerization reagent, (ii) a multimerization reagent, the multimerization reagent comprising at least two binding sites Z for the reversible binding of the binding partner C of the receptor binding reagent, wherein the receptor binding reagent (i) and the multimerization reagent (ii) form multivalent binding complexes that bind to said target cell, each multivalent binding complex comprising at least two of said receptor binding reagents bound to one said multimerization reagent, said multivalent binding complex providing increased avidity, relative to said receptor binding reagent; and (iii) said detectable label bound or capable of binding to said multivalent binding complex, wherein said target cell is a mammalian cell.