The in vivo effects of ethrane and single intravenous injection of hydrocortisone 250 mg on T-cell subsets (T3, T4 and T8), s-cortisol, s-Na+ and K+, polymorphonuclear leukocytes, lymphocytes and HSR (T-helpers to T-suppressors ratio) has been studied. Six healthy volunteers and 16 patients was selected and observed. Sixteen patients has been done surgery under ethrane anesthesia, and single dose of hydrocortisone 250 mg was injected intravenously to eight patients among them. The result is as follows; 1) Serum cortisol level was increased after surgery under ethrane anesthesia and was decreased bellow control value at 24 hours after injection of hydrocortisone 250 mg. 2) After surgery under ethrane anesthesia, PMN's counts and monocyte counts in peripheral blood were increased, but absolute number of lymphocytes and T-cell subsets were decreased. 3) After surgery under ethrane anesthesia with hydrocortisone 250 mg, there was the most elevation of PMN's counts and the most redution of absolute number of T-cell subsets at 5 hours and was somewhat tended to return to control values, but remained changed at 24 hours, and monocyte counts was unchanged. 4) Adding hydrocortisone 250 mg compared with ethrane anesthesia alone, the increasing rate of PMN's counts and the decreasing rate of lymphocytes and absolute number of T-cell subsets were higher at 5 hours, and were lower at 24 hours. 5) Relative percentage of T-cell subsete and HSR were unchanged after surgery under ethrane anesthesia alone and adding hydrocortisone 250 mg.