SummaryImmune reactivity was assessed by measuring skin reactions to 5 antigens and one mitogen on three consecutive days in a control population of 67 subjects and in patients with 12 different disease conditions.Highly significant differences between patient groups and controls in the means of the largest diameters or the frequency of positive skin tests were observed after 24 h. and, to a lesser extent, after 48 h. Abnormal skin reactions to tuberculin and trichophytin were only observed in a few patient groups, but when they did occur, differences from controls were highly significant (increased mean diameter in tuberculosis, total absence of tuberculin reactivity in Hodgkin disease, decreased trichophytin reactivity in sarcoidosis).On the basis of our observations, practical criteria for evaluating delayed cutaneous hypersensitivity can be proposed, and the value of standardized DCH in evaluating cellular immunityin vivoshould be reassessed.