Copper is an essential trace element micronutrient in human and animal nutrition. Trace amounts present even in ultrapure water, serum and other cell culture medium components are sufficient to support the health requirements of most cell types in culture. Analysis of a variety of different types of basal media from a number of different suppliers revealed large fluctuations in the levels of copper, and also of other micronutrients including zinc, iron, selenium and cobalt. Investigations on proliferating Caco-2 cells revealed reductions in growth with increasing copper concentrations within the range seen in the commercial media and changes in expression of apoptosis- and autophagy-related proteins were noted. Even at concentrations of 1 μM CuSO₄ where there was no significant change in cell growth, there was a significant decrease in procaspase-3 expression. These results stress the importance of batch testing of basal media when undertaking trace metal research since the baseline levels may vary. Batch variation of serum is well established but our results suggest that batch variation of the media may also be important.