Singlet oxygen lifetimes for detergent-dispersed L1210 leukemia cells in deuterium oxide buffer were measured by following the decay of 1270 nm phosphorescence. Four photosensitizers and two detergents were studied. Stern-Volmer plots were linear over the cell concentration range studied (0-10(7) cells/mL). The singlet-oxygen quenching constants obtained depended somewhat upon the specific combination of detergent and photosensitizer used. Extrapolation of the singlet-oxygen lifetime data to "100%" cell concentration (1.39 +/- 0.04 x 10(9) cells/mL) and correction for the contribution of the water solvent gave a singlet-oxygen lifetime between 0.17 and 0.32 microseconds for the L1210 leukemia cell. The theoretical contributions of various types of biological molecules within the L1210 cell to the total singlet-oxygen quenching were calculated from their concentrations and their quenching constants. These calculations suggest that proteins will quench most of the singlet-oxygen. Only about 7% of the singlet-oxygen is quenched by water.