Polyamines induce the transport in vitro of the rat liver precursor of ornithine transcarbamylase (pOTC) into isolated rat liver mitochondria. The accumulation of this precursor at the level of binding to the mitochondrial surface has allowed us to establish that polyamines are involved in the interaction of the precursor with the mitochondrial surface. Transport of a chimeric protein having the signal sequence of pOTC fused to a fragment of the cytosolic protein human arginosuccinate lyase was also induced by polyamines. The sensitivity of the pOTC synthesized in vitro and of the chimeric protein to proteinases decreases in the presence of polyamines. This result suggests that polyamines may play a role in modulating the folding of precursors to favour their binding to mitochondria.