Studies on the effect of thymosin on T-cell levels in vitro among normal persons and cancer patients show that, in general, T-cell levels increase after incubation with thymosin in populations with low initial T-cell levels while the levels decrease in populations with high initial T-cell levels. In patients with small cell carcinoma of the lung receiving intensive chemotherapy also randomized to receive thymosin at a dose of 60 mg/m2, thymosin at a dose of 20 mg/m2, or placebo twice weekly, increased survival occurred in patients receiving the thymosin dose of 60 mg/m2. The increase in survival was greatest in patients with low pretreatment T-cell and alpha2HS-glycoprotein levels. These observations suggest that the cancer patients most likely to benefit therapeutically from adjuvant treatment with thymosin are those with relatively low initial T-cell levels and other parameters of cellular immunity.