We measured the neopterin level in the supernatant of cultured alveolar macrophages from patients with interstitial lung diseases (ILD patients) as a marker for the activation of alveolar macrophage. 2. In ILD patients, the supernatant neopterin level (40.1 +/- 7.8 pmol/ml) was significantly higher (P < 0.01) than that in control subjects (10.0 +/- 1.6 pmol/ml). 3. We also found that macrophage-colony stimulating factor (M-CSF) and interleukin-2 (IL-2) augmented neopterin production from alveolar macrophage in both ILD patients (51.6 +/- 10.4 and 60.1 +/- 10.8 pmol/ml, respectively, P < 0.01) and control subjects (28.1 +/- 6.0 and 25.7 +/- 4.9 pmol/ml, respectively). 4. These findings suggest that alveolar macrophages produce neopterin by M-CSF or IL-2.