ESAT-6 from Mycobacterium tuberculosis is an important T-cell antigen for cell-mediated immunity in the early phase of tuberculosis infection. Since the lung is the organ in which infection is initiated, immune responses in the lung play a significant role in restricting the initial infection with M. tuberculosis. The aim of the present study was to assess whether efficient cell-mediated immune responses in the lung and draining mediastinal lymph nodes could be stimulated by pulmonary administration of ESAT-6 encapsulated in poly(lactide) (PLA) microspheres. BALB/c mice were immunised intranasally on days 1, 28 and 56 with 2 microg microencapsulated ESAT-6. Cellular responses in the lungs, spleen and mediastinal lymph nodes (MLN) were characterised using ELISPOT and proliferation assays. Fluorescence activated cell sorting (FACS) was used to assess the expression of CD44 on CD4+ and CD8+ cells derived from the MLN of immunised animals. For comparison, groups of mice were immunised intranasally with soluble 'free' ESAT-6 or intramuscularly with ESAT-6 in Alhydrogel. Intranasal instillation of microencapsulated ESAT-6 induced greatest numbers of ESAT-6 specific IFN-gamma and IL-4 secreting cells in the lung and MLN (P<0.05). Similarly, ESAT-6 specific recall responses were strongest following intranasal immunisation of mice with microsphere encapsulated antigen (P<0.05). FACS demonstrated a higher proportion of T cells expressing CD44 in the MLN from mice immunised intranasally with microencapsulated ESAT-6. These data support the notion that the immune system is compartmentalised and responses are often strongest in compartments proximal to the site of vaccine application. Furthermore, our data indicate that, for efficient activation of cell-mediated responses, antigens must be presented to the immune system in an appropriate formulation.