Abstract Spermidine, an aliphatic polyamine present in high concentrations in the white matter, could act as a bivalent ligand stabilizing myelin lamellae. To seek an answer to the title's question, polyamines were extracted from the subcellular fractions of rat brain after intracerebral injection of [ 14C]putrescine, a precursor of spermidine. Purified myelin contained a considerable amount of spermidine but only traces of spermine. All other fractions contained both polyamines in various proportions. The specific activity of myelin spermidine was much lower than that of any other fraction. Results support the idea of an intrinsic localization of spermidine in myelin, rather than contamination.