Human red blood cells (RBCs) are considered to play a significant role in both blood pressure (BP) regulation and tissue oxygenation, because they can bind as well as release previously bound nitric oxide (NO) from hemoglobin (Hb) and other intracellular components. Two reports indicate that the human RBC possesses nitric oxide synthase (NOS) activity-by the accumulation of nitrite across a membraned chamber in one and by the hydrolysis of labeled L-arginine, presumably to labeled L-citrulline, in the other. Furthermore, NOS proteins have been identified by immunoblot in RBCs. If true, the presence of NOS activity would convert the human RBC to a donor with limited ability to bind exogenously generated NO. In considering the importance of the question of the presence or not of NO synthetic capacity of this cell in BP regulation and tissue perfusion, whether human RBCs are, indeed, able to hydrolyze L-arginine to L-citrulline, the coproduct of NO was explored. RBC samples collected from control subjects were assayed for NOS activity by incubation of homogenized cellular fractions with labeled tritiated L-arginine in the presence of 0.5 mmol/L NADPH. By this method, the amino acid coproduct of NO, tritiated L-citrulline, would be recovered in the supernatant after removal of unused substrate by cationic resin treatment. At first, activity appeared to be present in the RBC supernatant but not in the pellet. However, activity was not suppressed by known inhibitors of NOS, whereas activity was suppressed by norvaline, an inhibitor of arginase activity with no known effect on NOS. By contrast, RBC arginase activity was not inhibited by N(omega)-nitro-L-arginine, NG-methyl-L-arginine, or aminoguanidine, known inhibitors of NOS, but was inhibited by norvaline. The label recovered by thin-layer chromatography was determined not to be tritiated L-citrulline but was instead tritiated L-ornithine, the product of arginase activity. Thus the enzymatic hydrolysis of L-arginine was not caused by NOS but was a result of the action of the enzyme arginase, which abounds in this cell. However, proteins interacting with antibodies to the endothelial and inducible isoforms of NOS were detected in human RBCs by immunoblot. Together, these findings indicate that human RBCs collected from normal adult individuals possess proteins that react with monoclonal antibodies to the Inducible and endothelial isoforms of NOS, but the proteins are without catalytic activity.