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1. Human leucocytes whose adenine nucleotide pool was prelabelled with [3H]adenine were investigated for their capacity to release adenosine and its metabolites and histamine when activated with the calcium ionophore A23187, antiimmunoglobulin E and the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (f-MLP). 2. Sixty-nine per cent of the 3H-label assimilated by the cells was incorporated into their adenine nucleotide pool in the ratio adenosine 5′-phosphate (AMP):adenosine 5′-pyrophosphate (ADP):adenosine 5′-triphosphate (ATP), 3:1:1. Spontaneous release of label from leucocytes plateaued at 5 min at 22.1 ± 4.2% of the total radiolabel incorporated and mainly consisted of hypoxanthine, inosine and adenosine. 3. Activation of cells with the calcium ionophore A23187 (1.0 μmol/l) caused a net increase in [3H]purine release above baseline of 27.9 ± 4.6% accompanied by a net basophil histamine release of 46.6 ± 9.4%. A23187 (1.0–3.0 μmol/l) caused a parallel concentration-dependent release of labelled purines and histamine. Purified mononuclear cells and granulocytes exhibited a similar ionophore-dependent capacity to release [3H]purines. 4. The distribution of label did not significantly differ between supernatants of activated and non-activated cells. Preincubation of cells with dipyridamole (10μmol/l) and erythro-9-(2-hydroxy-3)-nonyladenine (EHNA) (10μmol/l) to inhibit uptake and catabolism of adenosine respectively, produced an increase in adenosine at the expense of hypoxanthine recovered from the supernatant of both ionophore-stimulated and resting cells. 5. Activation of leucocytes with f-MLP (1.0μmol/l) caused a net increase in release of basophil histamine, but was associated with only a transient net increase in release of [3H]purine in four of five experiments. Immunoglobulin E-dependent stimulation produced basophil histamine release in the absence of detectable [3H]purine release. 6. These data demonstrate that leucocytes can be stimulated to release adenosine and related nucleoside matabolites accompanying the release of basophil-derived histamine. As adenosine is a bronchoconstrictor in asthma and leucocyte infiltration of the airways is a characteristic feature of the disease, leucocyte-derived adenosine may contribute to the pathogenesis of airways obstruction.