The report of Monjan and Collector (1) emphasizes the modulating effects of stress on the immune response in mice. This important and timely contribution strengthens a growing awareness that ei-ther intentional or uncontrolled anxiety-stress in various forms can have critical effects on immunological and pathologi-cal reactions in-mice and other experimental animals. There are some techni-cal matters intheir report, however, that require clarification. The authors' use of the terms" cortisol" and" cortisone" when referring to circulating plasma glucocorticoids in the mouse may have been inadvertent, but is, of course, scientifically incorrect. Unlike the case in man, dogs, rabbits, and certain other mammals that produce both cortisol and corticosterone, recent experiments have shown that in the mouse corticosterone is the only gluco-corticoid found in the plasma (2-4). Using a microfluorescence assay and a col-umn chromatographic separation procedure, we have demonstrated that cortisol is not present in plasma obtained from ei-ther normal quiescent mice or from mice that have been stressed in various ways. Even in pregnant mice where plasma concentrations of corticosterone were increased to 3600ng/ml, no coritsol was present (5). It is also unlikely that" cortisone" could be presentto act on lymphocytes since cortisone is not secreted by the adrenal gland and is normally found only in the liver of cortisol-producing mammals as a short-lived metabolite of cortisol (6).

The radioimmune assay (RIA) kit em-ployed by Monjan and Collector was de-signed for the measurement of cortisol in patients and other mammals where the predominant circulating glucocorticoid is cortisol. That this RIA cortisol proce-dure cross-reacts with corticosterone to an extensive degree is evident from their results. However, since the extent of the cross-reaction is unknown, it is difficult to deduce the actual plasma corticoste-rone concentrations based on the" cortisol" values which were reported. Additional items of information needed for a betterevaluation and appre-ciation of their important results are:(i) the time of day at which blood samples were obtained fromtheir stressed mice and (ii) the specific procedures employed in handling the mice and in obtaining blood samples. The fluctuating concen-tration of plasma corticosterone in the mouse follows a circadian rhythm, rang-ing between 5 and 35 ng/ml in the morn-ing and early afternoon but increasing to over 200 ng/ml between 7 and 10