The inhibition of the production of malonyldialdehyde (MDA) in guinea-pig lung homogenates, incubated in the presence of 50 microM arachidonic acid and 1.4 mM adrenaline, has been exploited as a simple and reliable assay to test in vitro non-steroidal anti-inflammatory agents (NSAIA). The inhibitory potencies of a series of reference NSAIA, which correlated fairly well with in vivo anti-inflammatory activity as determined by carrageenin oedema, are herewith reported. The specificity of the assay was also evaluated by testing up to forty miscellaneous drugs: none of these significantly reduced the MDA production.