A new methylxanthine derivative [7-(1,3-dithiolan-2-ylmethyl)-1,3-dimethylxanthine, ABC 99] with mucoregulatory and antibronchospastic properties has been studied. ABC 99 was observed to have marked anti-inflammatory activity in a series of experimental trials involving the principal mediators of inflammation (PAF, histamine, serotonin, LTC4-like substances, etc). It inhibits the formation of oedemas induced both by carrageenin and PAF in the rat paw, and reduces the increase in vascular permeability induced by histamine and serotonin. ABC 99 was also found to inhibit PAF-induced pleurisy, reducing the volume of pleural exudate and the presence of LTC4-like substances in the pleural cavity. When administered subacutely, ABC 99 checks the formation of granulation tissue caused by the subcutaneous implantation of cotton pellets in the rat. These experimental results indicate ABC 99 may be of particular interest in the treatment of respiratory disorders involving obstructive inflammation and bronchial hypersensitivity.