A dinitrochlorobenzene (D.N.C.B.)/red-blood-cell conjugate inhibited migration of leucocytes which came from D.N.C.B. sensitised patients. This effect provided the basis for a rapid, sensitive, and quantitative in-vitro measure of D.N.C.B. sensitivity. Frequent serial measurement of cell-mediated immunity was possible with the test, provided D.N.C.B. sensitivity was maintained by occasional skin patch tests.