Cultivated human glial cells, kept in a state of density-dependent inhibition of growth, accumulate age-pigment (lipofuscin) within their lysosomal vacuomes with the same characteristics as the corresponding pigment observed in vivo. The rate of formation and accumulation of lipofuscin is greatly accelerated under the conditions of routine cell cultivation in comparison to the in vivo event. Lipofuscin is generally considered to be composed of polymerized products of lipid peroxidation and thus it would be reasonable to suggest that factors which influence lipid peroxidation would also alter the rate of lipofuscin formation. Human glial cells were grown in the presence of various oxygen concentrations in the gas-phase (5%, 10%, 20%, 40%). This was found to modulate (accelerate or decrease) the rate of lipofuscin formation. The present study thus provides: important supportive evidence for the lipid peroxidation origin of lipofuscin, a useful model system for studying the effect of lipofuscin accumulation on lysosomal function and cell growth kinetics, evidence that our standard culture conditions are far from ideal since oxygen concentration may drastically alter rates of lipofuscin formation and accumulation. Cell culture technique, as we know it today, may benefit from more closely controlled oxygen tensions, i.e., by reducing oxygen to levels that more closely approximate conditions in vivo.