The expression of delayed-type hypersensitivity in animals has been inhibited by a variety of anticoagulants, but direct evidence for activation of clotting in the evolution of these reactions has been lacking. Using the fluorescent antibody technique we here demonstrate that fibrin deposition is a prominent and consistent feature of both allergic contact dermatitis and classic delayed hypersensitivity skin reactions in man. Fib was detected in 55 of 58 delayed reactions studied at the peak of their intensity. The characteristic distribution of Fib—principally in the intervascular portions of the reticular dermis with sparing of vessels and their associated cuffs of mononuclear cells—is unusual and quite different from that described in antibody-mediated lesions in animals or man. Fib was found in vessel walls in only 2 of 94 biopsies studied. With a single exception, deposition of immunoglobulins and complement was not observed.