Spermine was toxic to BHK-21/C13 cells in the presence of newborn calf serum and the toxicity, but not the metabolism of spermine, was prevented by aminoguanidine. Aminoguanidine treatment resulted in significant alterations in the polyamine profile of these cells with loss of intracellular putrescine after 4 hr of exposure. In the presence of aminoguanidine, intracellular polyamine content returned to control values at 24 hr, possibly as a result of increased uptake of exogenous spermine.