To the Editor.
—Phytohemagglutinin (PHA) is an agent mostly used for in vitro testing of lymphocyte transformation. Less attention has been paid to its possible use as a nonspecific skin antigen for the in vivo assessment of delayed-type hypersensitivity.1,2 Similarly to the common recall antigens, ie, PPD, streptokinase-streptodornase, candidin, and others, the intradermal inoculation of PHA causes perivascular cell infiltration consisting of both polynuclear and mononuclear cells, with some lymphocytes developing to blastlike cells.1In the February Archives (1982; 142:291-293), Burke et al, in an article entitled "Immune Responses to Varicella-Zoster in the Aged," comment on the skin reactions to intradermal injections of PHA in healthy volunteers and in immunologically compromised patients. As far as the healthy aged subjects are concerned, their results are fully in agreement with our investigation,3 carried out in a nursing home for the elderly. We have observed normal PHA skin responses even