- OBJECTIVE--To investigate the effect of heparin and thrombolytic agents on superoxide generation by human neutrophils, as inhibition of superoxide production may have a role in reducing ischaemia and reperfusion injury. METHODS--Neutrophil superoxide production stimulated by phorbol myristate acetate (PMA), opsonised zymosan, or formyl methionyl leucyl phenylalanine (FMLP) was measured as the superoxide dismutase inhibitable reduction of acetyl ferricytochrome c by a microtitre plate technique. RESULTS--Heparin, at concentrations of 0.5-500 U/ml, caused a gradual inhibition of superoxide production stimulated by PMA, opsonised zymosan, or FMLP. Tissue plasminogen activator was more potent than heparin in inhibiting superoxide production induced by opsonised zymosan or FMLP, but it did not affect the activity stimulated by PMA. Streptokinase or urokinase had no effect on superoxide production. When heparin was used in combination with tissue plasminogen activator, streptokinase, or urokinase at their therapeutic concentrations there was a significant inhibition of superoxide generation (70%, 30%, and 25%, respectively). The therapeutic concentrations of tissue plasminogen activator alone caused a reduction of 40% of neutrophil superoxide production. When tissue plasminogen activator and streptokinase were both added to neutrophils, however, a synergistic inhibition of 80% was achieved. CONCLUSIONS--The inhibition of super oxide generation by these drug combinations may explain the limited inflammatory response and reduction of reperfusion injury observed in patients receiving thrombolytic treatment.