The number of lymphocytes with mobile receptors for concanavalin A (Con A) on their surface membrane (forming visible caps after the addition of fluorescein-conjugated Con A) was determined in the peripheral blood of 53 patients with Hodgkin disease. Of 29 individuals studied prior to treatment, the level of capped cells was found to be below the normal range in 9 of 13 in stages I and IIA, 6 of 8 in stage IIIA, and all 8 in stages IIIB and IV. Even among patients in remission 2 yr after successful treatment the level was below the lower normal limit in 9 of 16. The number was also reduced in 7 of 8 individuals with recurrent lymphoma. The level of lymphocytes that cap with Con A may prove to be a more sensitive measure of active Hodgkin disease than the total peripheral lymphocyte count or the level of T cells. This lymphocyte parameter merits further study as a correlate in vitro of cellular immunity.