Alveolar macrophages (AMs) from patients with interstitial lung diseases, such as sarcoidosis and idiopathic pulmonary fibrosis, suppress the phytohaemagglutinin (PHA) stimulation of autologous peripheral lymphocytes. The aim of this study was to determine whether the suppressive effect of alveolar macrophages of patients with interstitial lung disease is due, not only to the secretion of soluble factors prostaglandin E2 (PGE2), interleukin-1 (IL-1) but is also correlated to a direct effect of AMs on the expression of IL-2 receptors (IL-2R: CD25) and on the induction of IL-2 activity. We studied 26 subjects, 8 with sarcoidosis, 7 with idiopathic pulmonary fibrosis, and 11 controls. Alveolar macrophages of sarcoid and idiopathic pulmonary fibrosis patients suppressed proliferation of autologous peripheral lymphocytes by 68 +/- 14% and 53 +/- 4.5%, respectively, compared to enhancement of 19 +/- 11% in three controls and suppression of 25 +/- 11% in the other six controls; the difference between subjects with interstitial lung disease and controls was significant. As already reported, the alveolar macrophages of sarcoid patients secreted large amounts of IL-1 (184 +/- 59 U.ml-1) whereas the alveolar macrophages from idiopathic pulmonary fibrosis patients secreted large amounts of PGE2 (3.6 +/- 2 ng.ml-1 x 10(-5) cells) compared with 23 +/- 19 U.ml-1 IL-1 and 0.34 +/- 0.15 ng.ml-1 x 10(-5) cells respectively, of controls. Suppression by supernatants recovered from lipopolysaccharide (LPS) stimulated alveolar macrophages can only partially explain the high suppressive effect of alveolar macrophages of interstitial lung diseases.(ABSTRACT TRUNCATED AT 250 WORDS)