Addition of a physiological concentration of NaCl (0.9%) to 0.05 mol/l Tris buffer (pH 7.4) reversed the enhancement of the activation of glutamic-type plasminogen (Glu-Plg) by low molecular weight urokinase by fucoidan and heparin, while addition of aminohexanoic acid (6-AH) enhanced the activation. Native fucoidan (N-2), sulfated fucoidan (S-2) and heparin alone and in the presence of 6-AH were investigated to determine the effect of NaCl addition to 0.05 mol/l Tris (pH 7.4) on the activation of Glu-plg by high molecular weight urokinase (HMW u-Pa). Heparin alone and in conjunction with 6-AH enhanced 4.5 to 5.5-fold the initial rate of activation of Glu-plg, while N-2 alone or in conjunction with 6-AH gave 3 to 5.5-fold enhancement and S-2 gave no enhancement of activation using 0.05 mol/l Tris buffer (pH 7.4). Addition of 0.9% NaCl to the buffer reversed the enhancement by the cofactors but, in the presence of 6-AH, N-2 gave two-fold and S-2 gave three-fold enhancement of activation while heparin gave 25% enhancement. The mechanism of enhancement by S-2 in the presence of 6-AH was investigated by dilution and enzyme kinetic studies. The results show that the enhancement was due to interaction with HMW u-PA and not with Glu-plg.