To the Editor:— In "Cat-Scratch Disease" ( 207 :312, 1969), the authors used the criterion of erythema of 1 cm or more for 48 hours after injection of 0.1 cc of antigen intradermally. "Induration has not been as useful as erythema for interpreting the cat-scratch skin test." The data from our laboratory suggest that erythema is a poor way of defining a positive skin test to catscratch antigen. Warwick et al ( Proc Soc Exp Biol Med 93 :253, 1956) have quite clearly documented evidence that the major reactant to the skin test antigen is the migration of mononuclears and that the basis for a positive reaction must of necessity rest on this phenomena. Our major criterion has been induration of 5 mm or more at 48 hours. Frequently, erythema has not been helpful. A great discovery is a fact whose appearance in science gives rise to shining ideas, whose light dispels