Calves, sensitised by cutaneous application of dinitrochlorobenzene, developed lesions of delayed hypersensitivity at the sites of application nine to 14 days later. Leucocytes from sensitised calves, collected 10 to 16 days after sensitisation, reacted in an in vitro leucocyte adherence inhibition test with the same hapten in water soluble form, dinitrobenzenesulphonate. The leucocyte adherence inhibition microtest which was simple, rapid, reproducible and amenable to rigorous statistical analysis, should find use in many areas of bovine immunology.