The interactions between a yeast cytosolic hsp70, Ssa1p, and various synthetic peptides, including mitochondrial presequences, have been studied. The interactions were monitored both indirectly, by measuring the effects of the presequences on the ATPase activity and oligomeric state of the enzyme, and directly, by measuring the increased steady-state fluorescence polarization of fluorescent derivatives of the presequences as they bind to Ssa1p. The presequences are all able to convert Ssa1p from an oligomeric to a monomeric form in a concentration-dependent manner. The presequences are also able to stimulate the ATPase activity of the enzyme at similar concentrations. Quantification of the binding by fluorescence polarization showed that the affinity for Ssa1p is directly related to the physical properties of the presequences. The most amphiphilic presequences, as measured by retention times on reversed-phase high pressure liquid chromatography or surface activity in lipid monolayers, had the highest affinity for Ssa1p. The least amphiphilic presequences, which had previously been shown to be ineffective as mitochondrial targeting sequences, had relatively low affinity for Ssa1p. The results show that Ssa1p interacts with a broad range of amino acid sequences and that the strength of these interactions is related to the physical properties of the sequence. That the physical properties recognized by Ssa1p are identical to those necessary for the targeting function of mitochondrial presequences suggests that Ssa1p may interact with mitochondrial precursor proteins in the cytosol. The interactions may serve a variety of purposes: the maintenance of precursors in translocation-competent forms, the prevention of improper association of precursors with non-mitochondrial membranes, and the delivery of precursors to the mitochondrial surface.