Fibrin, not fibrinogen, enhances the rate of tPA catalysed plasminogen activation. In earlier studies we have shown that a site involved in this rate enhancement is located in a tridecapeptide, i.e. fibrinogen A alpha-(148-160). This sequence comprises a special charge distribution in which a stretch with alternating neutral and acidic amino acids is embraced by basic amino acids. In this study we found that the disruption of charge distribution as caused by replacing valine 152 by other (charged and/or polar) amino acids leads to loss of rate-enhancing capacity. Also lysine at position A alpha-157 was replaced by lysine derivatives and other amino acids. We found that the side chain of the amino acid at position A alpha-157 must contain no (as in glycine) or one carbon atom without substitution (alanine). When the side chain contains two or more carbon atoms, there should also be a polar group in the side chain. We also synthesized a series of hexapeptides covering the sequence of A alpha-(148-160), and found that only A alpha-(154-159) is stimulatory, notwithstanding the fact that the peptides A alpha-(152-157), A alpha-(153-158) and A alpha-(155-160) also contain lysine A alpha-157. We conclude that the shortest peptide with stimulation activity is A alpha-(154-159); that the charge distribution in A alpha-(148-160) is important; that it is not lysine A alpha-157 per se that is crucial, but rather the properties and orientation of the side chain of A alpha-157.