The development of T-cell responses in pigs vaccinated against Aujeszky's disease in the presence of maternal-derived antibodies (MDA) was examined. The aim of study was to evaluate the influence of MDA on the postvaccinal T-cell responses and optimization vaccination protocols in MDA-positive pigs. Pigs born to immune sows were vaccinated at different ages against Aujeszky's disease virus (ADV). For estimation of T-cell responses the lymphocyte proliferation and interferon (IFN)-gamma and interleukin-4 production were evaluated. High values of stimulation index were noted in groups vaccinated at 8 or 12 weeks of age (in 60% and 100% animals, respectively). In weaners vaccinated at 10 and 14 weeks of age, as well as in those vaccinated at 7 days and revaccinated at 8 or 12 weeks of age, 100% of animals positively responded in the lymphocyte proliferation assay after booster. At 20 weeks of life, only animals vaccinated at 12 weeks of age, 7 days and 12 weeks of age, and 10 and 14 weeks of age showed antigen-specific proliferation. Similar results were observed with IFN-gamma secretion after exposure to live ADV. We demonstrate that early vaccination with a live glycoprotein E-deleted ADV vaccine, in the face of high levels of MDA, could be effective, but the intensity and duration of the anamnestic response depends on the time of booster injection.