The pH in phagolysosomes of rabbit alveolar macrophages was studied using yeast particles labeled with fluorescein isothiocyanate (FITC). The yeast particles were added to the macrophages in vitro a few hours or 1 day after they had been lavaged from the lung and in vivo 3 h or 1 day before the lungs were lavaged. Intracellular pH was estimated from the ratio between the fluorescence intensity at wavelength 519 nm with excitation at wavelengths of 495 and 450 nm. In both the in vitro and in vivo experiments pH decreased significantly during the first hours after lavage, but after a few hours reached almost the day-2 levels, i.e., 4.9-5.4. The decrease in pH was related to time after lavage and not to time after phagocytosis of the particles. It is suggested that intracellular measurements of pH in alveolar macrophages should be combined with determinations of lung clearance of metal particles.