An acetone-soluble fraction of human dander provoked positive patch test reactions in 45 (38%) of 120 patients with atopic dermatitis. The positive patch tests were rare in normal controls and clinical controls. The positive patch test reactions histologically showed spongiosis and mononuclear cell migration in the epidermis, edema and mononuclear cell infiltrate in the upper dermis. Scratch tests with the acetone-soluble human dander fraction were positive in only 2 (2%) of the 120 patients examined. It was suggested that in a considerable number of patients with atopic dermatitis, an acetone-soluble fraction of human dander provokes a delayed-type skin reaction without inducing an immediate-type skin reaction.