1. We describe a rapid and reliable technique for the assessment of basal nitric oxide release in clinical situations, using peripheral blood polymorphonuclear leucocytes isolated by a single-step density gradient procedure. The assay is based on the quantitative conversion of oxyhaemoglobin to methaemoglobin by nitric oxide. We have further examined the ability of these cells to respond to various stimuli.

2. Basal (unstimulated) nitric oxide release occurred, which was augmented by superoxide dismutase. The mean value for healthy subjects was 283 ±96.7 pmolmin−1 10−6 cells.

3. Both phorbol myristate acetate and N-formyl-methionyl-leucylphenylalanine induced further release of nitric oxide, which was increased by preincubation with lipopolysaccharide, interleukin-6 and interferon-γ.

4. Preincubation of cells with NG-monomethyl-l-arginine or l-canavanine sulphate inhibited nitric oxide production.

5. The procedure provides a valuable tool for monitoring nitric oxide up-regulation in clinical situations.

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