A Clark-type microelectrode is used to measure oxygen consumption rates in laser-irradiated solutions of photosensitizer and photosensitizer-containing cells. The presence of a singlet oxygen-specific acceptor molecule, furfuryl alcohol, permits indirect determination of relative singlet oxygen generation efficiencies from oxygen consumption data. Solution and cell measurements are performed which compare photosensitizing efficiency of Photofrin-II (PII), tetraphenylporphine tetrasulfonate (TPPS4), mono-L-aspartyl chlorin e6 (MACE), and chloroaluminum sulfonated phthalocyanine (CASPc). Relative singlet oxygen generating efficiency, per-unit-weight and per-absorbed-photon, were determined to be: MACE > CASPc > TPPS4 > PII and TPPS4 > MACE > PII > CASPc, respectively. When these results are compared to oxygen consumption in photosensitizer-containing cells, differences in the order and magnitude of photosensitizing efficiencies are observed. The relative oxygen consumption rate in cells was: PII CASPc > MACE TPPS4. Additional information concerning cell killing efficiency is derived from clongenicity assays. These data indicate that consideration of singlet oxygen generating ability in solution must be considered in conjuntion with cellular assays in order to provide an in vitro estimate of photosensitizer efficacy.