The purpose of this article is to study the amounts of gamma-aminobutyric acid-transaminase (GABA-t) during immune response in the human thymus. GABA-t was studied by biochemical and histochemical methods in 7 immunostimulated (treated) and 7 non-immunostimulated (untreated) patients (4 young adult, age range: 24-36 years; 3 older adult, age range: 56-66 years). Immunostimulation was performed using interferon drugs for 6 months. After the histoenzymatic staining of GABA-t activity, the slides containing the samples of thymus of treated and untreated patients underwent quantitative analysis of images. The present results provide direct evidence that the immune response increases the level of GABA-t contained in vessels, parenchyma and nerve fibers of the thymus. Treatment with interferon is also capable of increasing the protein content of the thymus. The biochemical data together with the histoenzymatic results provide evidence for a localization of GABA-t in the thymic gland. Moreover, gamma-aminobutyric acid can be considered as one of the linking molecules between the immune and nervous functions of the human thymus.