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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.

Serum thymic hormone activity was measured in 36 patients with myasthenia gravis and in 10 control subjects from each age decade. In all 25 patients under 50 years of age results were within, or close to, the normal range. Activity at levels considered normal for juveniles was detected in 10 of the 11 older patients whereas levels normally decline in older subjects. One week after thymectomy, 13 of 17 patients (76 per cent) had no demonstrable serum thymic hormone activity. However, 10 months or longer after thymectomy only five patients (30 per cent) lacked thymic hormone activity in the serum. There was a significant correlation between clinial improvement and sustained lowering of serum thymic hormone activity after thymectomy.

Circulating thymic-hormone activity was assayed by measuring Thy 1-2 antigen induction on null lymphocytes from athymic mice incubated with human plasma or serum. Plasma from 19 normal children aged under 10 had inductive activity equivalent to 10-6-16-2 ng thymopoitin/ml. Plasma from 15 infants were severe combined immuno-deficiency, 2 of whom had appreciable immunoglobulin synthesis, and from 2 infants with DiGeorge syndrome had little or no inductive activity. Successful reconstitution with thymus or bone-marrow grafts and with red-cell infusions (if adenosine-deaminase deficiency is present) was followed by a rise in circulating thymic-hormone activity.