Mammalian cells attached to membrane filters or deposited on filters without attachment were exposed to gas-phase singlet oxygen (1O2) in the absence of any other reactants. Cells were exposed in a monolayer or less, in the absence of external medium, during steady-state 1O2 generation, ensuring that singlet oxygen impinged directly and equally on all cells simultaneously. The current methodology for cell exposure ensures that 1O2 is initially the only reactive species to which the cells are exposed. Results seen with this system can therefore be attributed solely and unambiguously to events initiated by 1O2. Further, all cells in the sample receive the same magnitude of exposure per surface area per time interval, which supports calculations of the amount of 1O2 required for irreversible cell damage, based on measured 1O2 flux and exposed cell surface area. Exposure to pure 1O2 irreversibly damaged a variety of cell types, including rat basophilic leukemia, human squamous carcinoma and Chinese hamster lung fibroblast cell lines, and murine primary hepatocytes. Cell survival curves following exposure to 1O2 followed apparent first-order kinetics. A large number of singlet oxygen collisions (approximately 10(12)-10(13) were required to inactivate a cell, on average, indicating a low probability that singlet oxygen collision will reduce cell survival. Regardless of cell type or the survival endpoint measured, lethal toxicity required a fairly constant number of 1O2 collisions per cell. This poses a serious caveat in the assignment of causality in correlating 1O2-initiated cellular damage with mechanism of death, i.e. most damage observed will not be related to death.(ABSTRACT TRUNCATED AT 250 WORDS)