While progress in achieving quantitative measures of cellular immune responses has been rapid since the advent of various in vitro correlates of cell-mediated immunity (1-6), the need for a quick and precise measurement of such responses in vivo remains . Such measures as mean survival time after injection of immunocompetent cells into adult allogeneic irradiated hosts (cf. 7), for example, or mean graft survival time after skin transplantation suffer from excessive variability, length of time required for completion of the assay, and procedural difficulties . The induction of splenomegaly in neonatal recipients of allogeneic cells (8) is reasonably quantitative but operationally restrictive . Other assays such as enlargement of draining lymph nodes (9) or hepatic lymphoid infiltration (10), although more sensitive and more directly reflecting numbers of effector cells, are limited in applicability; the latter, moreover, requires histology. Local graft-vs .-host reactions have also been assessed by damage to the host kidney after injection of allogeneic immunocompetent cells under the kidney capsule (11), and by skin reactions elicited by intradermal inoculation of such cells (12) ; the latter has been called the normal lymphocyte transfer reaction (12, 13).The normal lymphocyte transfer reaction seemed to us to have the greatest potential advantage for an in vivo assessment of immunological disparity between donor and host. The extensive studies of Brent and Medawar (12,13) and of Billingham and his colleagues (14, 16) established the reaction as reflecting a local graft-vs .-host reaction . Their assessment of the strength of reaction was based on measurements of a local swelling, induration of the skin and erythema ; other adjuncts of inflammation have also been reported (17, 18) . Our own work has been directed at assessment of the host local vascular response which under appropriate experimental conditions can serve as a rapid, reliable, and quantitative measure of the activity of intracutaneously injected immunocompetent lymphocytes.