Soluble fibronectin of human plasma was specifically dissociated at neutral pH from gelatin-agarose by several cationic amino compounds, notably the polyamines spermine, spermidine and putrescine, the basic amino acid arginine, and amino sugars. The neutral and acidic amino acids and the N-acetylated derivatives of amino sugars tested were ineffective. Gel-filtration experiments demonstrated that [14C]spermidine bound to fibronectin but not to gelatin.