1. This study aimed to evaluate the effects of phosphodiesterase (PDE) inhibitors and currently prescribed anti-asthma drugs for their ability to inhibit inflammatory cell activation in vitro. 2. Alveolar macrophages and eosinophils were isolated from the bronchoalveolar lavage (BAL) fluid of ovalbumin (Ovalb)-sensitized guinea-pigs. Opsonized zymosan (OZ) and PAF stimulated leukotriene B4 (LTB4) release from eosinophils was measured by radioimmunoassay. Ovalb-induced superoxide generation was measured by reduction of cytochrome C. 3. Monocytes were separated from human peripheral venous blood and mast cells were dispersed from human lung fragments. Lipopolysaccharide (LPS)-induced tumour necrosis factor-alpha (TNF-alpha) release from monocytes was measured by ELISA and anti-IgE stimulated histamine release from mast cells was measured by a radioenzymatic method. 4. The beta 2 agonist, salbutamol inhibited TNF-alpha release from monocytes and histamine release from mast cells whilst having no effect on eosinophil-derived LTB4 release or macrophage superoxide generation. 5. The PDE 3 inhibitor, milrinone produced a concentration-related inhibition of TNF-alpha release from monocytes which achieved statistical significance at 10(-5) M but inhibited LTB4 release from eosinophils and superoxide generation from macrophages only at the highest concentration (10(-3) M) examined. Milrinone had no effect on histamine release from mast cells. 6. The selective PDE 4 inhibitors, denbufylline and rolipram and the corticosteroid, beclomethasone produced a concentration-related inhibition of LTB4 release from eosinophils, TNF-alpha release from monocytes and superoxide generation from alveolar macrophages whilst having no effect on histamine release from mast cells. 7. The mixed PDE 3/4 inhibitor, benzafentrine produced a concentration-related inhibition of LTB4 release from eosinophils, TNF-alpha release from monocytes, superoxide generation from alveolar macrophages and histamine release from mast cells. 8. In conclusion these data clearly show that both established anti-asthma medication as well as PDE inhibitors have the potential to inhibit inflammatory cell activation in vitro but that the anti-secretory actions of beta 2 agonists, corticosteroids and PDE inhibitors are distinct.