Hydrocortisone at 10––4M concentration could inhibit the response of human lymphocytes to galactose oxidase stimulation. Removal of the hydrocortisone after 4 h of culture did not reverse this inhibition. On the other hand, removal of monocytes greatly enhanced it, so that it could be observed at 10––6M concentration. Next, tonsil cells were separated into two fractions relatively rich in either T or B cells. Both fractions could respond to galactose oxidase and the response could be inhibited by hydrocortisone.