Serum thymic factor (STF) activity was assayed in 26 young asthmatic patients and in 20 age-and sex-matched controls, in view of the increasing importance attributed to this thymic product in regulating several immune mechanisms. Determinations of total and specific IgE and of other immunoglobulins and skin tests were also performed. Decreased STF activity and high total and specific IgE concentrations were observed in the majority of asthmatic patients. In controls, STF activity was within normal limits in all cases and total and specific IgE concentrations were increased only on one case. The decreased STF activity may be responsible for a possible impairment of cell-mediated immunity and for the increased stimulation of the reaginic system observed in asthmatic patients, via its effects on some T cell subpopulation, namely, suppressor T lymphocytes, which seem particularly sensitive to STF variations. On the basis of these data and of the results of our study, a possible, although speculative, correlation between reduced STF activity and high IgE concentrations in asthmatic patients may be postulated. The reported increased occurrence of autoimmune phenomena in asthmatic patients is in agreement with the hypothesis of a STF-mediated suppressor T-lymphocyte quantitative and/or qualitative defect in this disease.