The stoichiometry of animal cell cultures is examined with respect to its variation and suitability for process monitoring and control. In addition to the two often used stoichiometric ratios, i.e., lactate yield from glucose (Lac/Glc) and ammonium yield from glutamine (NH4(+)/Gln), five other less well characterized ones, i.e., ammonium yield from the total consumption of amino acids (NH4+/TAA), consumption of total amino acids to glutamine (TAA/Gln), essential amino acids to glutamine (EAA/Gln), glutamine to glucose (Gln/Glc), and oxygen to glucose (OUR/Glc), are also considered. A comparison of a number of cell lines including hybridoma, BHK, and CHO cells under a wide range of experimental conditions revealed that all the cell lines have similar patterns of variation of stoichiometry. In steady states of continuous culture, Lac/Glc and Gln/Glc are primarily determined by the residual glucose concentration while TAA/Gln and EAA/Gln correlate well with the residual glutamine concentration. Ammonium formation not only is a function of glutamine concentration but also is affected by the consumption of other amino acids, particularly at low residual glutamine concentrations. NH4+/TAA turned out to be a more suitable parameter to describe the ammonium formation. Large variations of all these stoichiometric ratios are found under conditions of relatively low residual concentrations of glucose and glutamine (both ca. < 0.2-0.5 mM). Above these concentrations the stoichiometric ratios are relatively constant and are independent of the cell lines. Thus, the correlations for these stoichiometric ratios may be directly used to control the nutrient concentration at low levels which are otherwise on-line difficult to determine. A stoichiometric equation is also derived for oxygen consumption. It is found that the metabolism of amino acids can significantly contribute to the consumption of oxygen. A correlation is obtained for OUR/Glc which may be used for the monitoring and control of mammalian cell cultures.