The correlation between leukocyte migration-inhibition and delayed skin hypersensitivity was studied in the sera of 15 anergic untreated Hodgkin's disease patients before and after administration of thymostimulin. Skin tests, negative in all patients before treatment, turned positive in 9 after thymostimulin. Prior to treatment, 10 patients had normal and 5 enhanced migration; none was leukocyte migration-inhibition factor (LIF) positive (mean 1.14 +/- 0.30). Seven of the 9 patients (77.9%) whose skin tests became positive, 3 of whom had an initially enhanced migration, were LIF positive after thymostimulin. LIF closely paralleled skin hypersensitivity both before and after thymostimulin. It is suggested that thymostimulin evokes the differentiation of a lymphocyte subpopulation(s) responsible for both factors investigated.