Lymphoid and non-lymphoid subpopulations were investigated in the lung of the mouse with immunocyto-, immunohisto- and enzyme-histochemical methods. Special attention was paid to the cell populations in bronchus-associated lymphoid tissue (BALT), which is positioned between a bronchus and an artery. In BALT, discrete T- and B-cell areas can be found. The majority of the T cells belong to the L3T4+ (T-helper) subpopulation. In the T-cell area interdigitating cells can be recognized by anti-class II antibodies as well as by specific monoclonal antibodies, NLDC-145 and MIDC-8. Macrophage subpopulations can be discriminated by location, enzyme reactivity and various macrophage-specific monoclonal antibody markers. On the outer rim of BALT macrophages are recognized by the MOMA-1 and ERTR9 antibody. Macrophages dispersed in BALT can only be discriminated with the MOMA-2 antibody. The macrophage markers F4/80 and Mac-1 show no reactivity in BALT. In lung, tissue macrophages around bronchi and blood vessels are predominantly recognized by the MOMA-1 and MOMA-2 antibody, and a minor population by the ERTR9 antibody. Alveolar macrophages show heterogeneity with the MOMA-1, MOMA-2 and NLDC-145 antibody. The relationship between alveolar macrophages and antigen-presenting cells is discussed here.