Tumor necrosis factor-alpha inhibited growth of cultured MCF-7 human breast cancer cells in a dose dependent manner. Tumor necrosis factor-alpha also markedly increased glucose consumption, and its cytotoxicity was modified by glucose concentrations in the growth medium; higher glucose levels were associated with increased cell survival. However, when the cells were perfused in physiological conditions, very high levels of tumor necrosis factor-alpha (200 ng/ml) in the perfusion solution had no inhibitory effects. Moreover, tumor necrosis factor-alpha had no effects on 31P nuclear magnetic resonance spectra of the perfused cells. In the traditional growth inhibition assays, cells are incubated for several days with a drug, a situation where their metabolism is altered due to the depletion of nutrients, the accumulation of toxic waste materials and pH changes. Perfusion experiments are more relevant to in vivo conditions, and may be used for studying metabolic processes and the mechanisms of action of therapeutic agents.