To investigate the mechanisms accounting for the T cell alveolitis taking place in the lung of HP patients and their pattern of growth, cells recovered from the bronchoalveolar lavage (BAL) of seven patients were evaluated for: 1) the expression of activation markers, including IL-2R (p55 and p75 subunits), HLA-DR and VLA-1 Ag; 2) the ability of IL-2 and IL-4 to induce in vitro proliferation; 3) the capability to synthesize and release IL-2 by determining the levels of IL-2 in BAL cell-free supernatants and by evaluating the presence of mRNA transcripts for IL-2; and 4) the molecular configuration of the beta- and gamma-genes of the TCR.