Antigen preparations extracted from Mycobacterium tuberculosis by different procedures--PPD and 'New tuberculin'--were compared with respect to the cytology of the Type IV reaction they elicit in the dermis. The numbers and microanatomical localization of cells of the major lymphocyte and monocyte subsets were measured histometrically in immunocytochemically-stained sections of biopsies of the 48 h reactions. Despite the close similarity in size of the external appearance of the reactions provoked by the two preparations, 'New tuberculin' elicited a more extensive perivascular infiltrate and more numerous diffusely infiltrating M3-bearing cells: the number of diffusely infiltrating CD4 and CD8 lymphocytes was similar in reactions against the two antigen preparations. Thus the method used to prepare a skin test reagent may affect the nature of the cellular reaction it provokes: differences in content of various classes of antigenic macromolecules are probably important, but the relative content of other irritant constituents may also be important.