We measured the amounts of tyrosine and 3-nitrotyrosine (NO2-tyrosine) in proteins of plasma and polymorphonuclear leukocytes (PMN) from human whole blood before and after activation with phorbol ester (PMA) or calcium ionophore (A 23187). In unstimulated blood, no significant nitration of tyrosine was detected into PMN proteins, but a NO2-tyrosine/tyrosine ratio of 0.7% was detected in plasma proteins. When blood was activated with PMA, the NO2-tyrosine/tyrosine ratio stayed at 0.7% in plasma proteins, but it increased to 1.4% in PMN proteins, indicating a peroxynitrite production within the cells. In blood activated with calcium ionophore, the NO2-tyrosine/tyrosine ratio was 1.2% in plasma proteins and 2.1% in PMN proteins. Incubation of blood with a NO-synthase inhibitor before stimulation inhibited such a protein tyrosine nitration. To ensure that NO2-tyrosine detected in intracellular proteins did not result from the enzymatic posttranslational tyrosylation of PMN proteins, the incorporation of 14C labeled tyrosine into PMN proteins after activation with PMA or A23187 was studied. The addition of a 10 fold excess of NO2-tyrosine did not modify the course of protein tyrosylation. Because tyrosine nitration is an irreversible reaction, NO2-tyrosine could be accumulated into proteins and could act as a cumulative index of peroxynitrite production.