An inhibitor of thymic hormone activity in serum from patients with lymphoblastic leukemia

Serum from 21 patients with lymphoblastic leukemia, five with myeloblastic leukemia and 30 age matched control subjects tested for thymic hormone activity in an assay that measures the induction of T cell surface antigen. This activity was subnormal in serum from 10 of 16 patients with untreated lymphoblastic leukemia (p less than 0.001) but was within the normal range when the leukemia was in remission. Low inductive activity was associated with an inhibitor of T cell induction which was less than 30,000 daltons in molecular size and interfered with induction by purified thymopoietin plus a high concentration of ubiquitin or by normal serum alone.