Lipid vesicles (liposomes) containing pH-sensitive fluorophores were used as probes for the study of liposome entry and intracellular fate. Pyranine [8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS)] was entrapped in the liposome aqueous core during preparation to provide a means of detecting internalization into living cells. HPTS is highly water soluble and shows a strong pH-dependent shift in its fluorescence excitation spectrum. Fluorescence emission (FEM) is slightly pH dependent with excitation (lambda EX) at 350-415 nm but highly pH dependent with lambda EX at 450 nm. Liposomes bearing a net negative charge bound rapidly to CV-1 cells and underwent endocytosis. One hour after liposome addition, high FEM with lambda EX at 413 nm and low FEM with lambda EX at 450 nm suggest that most cell-associated liposomes had been internalized and resided at a mean pH of approximately 6.6. Collapse of cellular H+ gradients with NH4Cl or monensin treatment rapidly and reversibly increased FEM with lambda EX at 450 nm. Direct examination by fluorescence microscopy corroborates the fluorometric data on internalization; over time, FEM remained high with lambda EX at 350-405 nm but decreased with lambda EX at 450-490 nm, showing that all lipid vesicles were internalized within 40 min at 37 degrees C. Acidification of intracellular liposomes increased over 3 h, reaching a minimum value of approximately pH 5.5. HPTS persisted within acidic cellular vesicles for 2-3 days, and cytoplasmic dye was observed infrequently, suggesting that liposome fusion with cellular membranes seldom occurs. Material delivered to the endocytic pathway via lipid vesicles labeled an assortment of intracellular organelles of varying motility and morphology, including dynamic tubular structures whose lumen is acidic.