O-2 uptake rates of animal cells (Chinese hamster ovary-CHO) were measured in 96-well microtiter plates by integrating with fluorescent sensors thereby measuring fluorescence intensity ratios of an O-2-sensitive and an insensitive fluorophor. O-2 consumption rate was estimated from measured dissolved O-2 and from O-2 mass transfer coefficient determined in advance. Specific uptake decreased with time from 3.2 x 10(-13) mol O-2 cell(-1) h(-1) at 15 h cultivation to 1.8 x 10(-13) mol O-2 cell(-1) h(-1) at 48 h. Specific O-2 uptake was also determined by sampling from a spinner-flask culture giving identical values. A cell viability assay for cultures based on O-2 measurements is described in which cells are incubated outside the fluorescence reader and then the dissolved O-2 is measured only once at a fixed time after the start of incubation. This protocol can be directly applied for high-throughput measurements.