The chemokine "regulated on activation, normal T-cell expressed and secreted" (RANTES) is a potent eosinophil and lymphocyte attractant with particular preference for CD45RO+ T-cells and eosinophils. These cells accumulate in the lungs of patients with sarcoidosis and fibrosing alveolitis. The purpose of this study was to determine whether RANTES mediates the inflammatory cell influx in these diffuse lung diseases. Cell types and number of bronchoalveolar cells expressing RANTES protein were investigated by immunocytochemistry using lavage cells obtained from 22 patients and 11 control subjects. Subsequently, RANTES messenger ribonucleic acid (mRNA) was semiquantitated using reverse transcription polymerase chain reaction (RT-PCR) methodology in unseparated lavage cell pellets in 26 patients and 13 control subjects. Cells expressing RANTES mRNA were identified by in situ hybridization. RANTES protein expression in lower respiratory tract (LRT) cells was identified in all study groups. The percentage of RANTES+ lavage cells in sarcoidosis was higher than in controls. RANTES was localized in the cytoplasm, mainly in alveolar macrophages (CD68+ cells) in sarcoidosis, and both in alveolar macrophages and eosinophils in fibrosing alveolitis. The same cell types which expressed RANTES protein expressed RANTES mRNA, as assessed by in situ hybridization. Sarcoidosis patients had higher levels of RANTES mRNA than the other groups. RANTES protein was higher in individuals with abnormal lymphocyte numbers: RANTES protein and mRNA expression was significantly correlated with lavage CD45RO+ lymphocyte numbers. These results indicate that RANTES may mediate T-lymphocyte influx in diffuse lung disease, particularly sarcoidosis. Moreover, they suggest that the cellular source of RANTES is the alveolar macrophage in sarcoidosis, and both macrophages and eosinophils in fibrosing alveolitis.