Many non-steroidal anti-inflammatory drugs (NSAIDs) (including sulphasalazine, sulindac, indomethacin, naproxen, salicylic acid, ibuprofen, piroxicam and mefenamic acid) were found to be competitive inhibitors (with respect to folate) of avian liver phosphoribosylaminoimidazolecarboxamide formyltransferase (AICAR transformylase, EC 126.96.36.199) and bovine liver dihydrofolate reductase (EC 188.8.131.52). In contrast, aspirin and the antipyretic-analgesic drugs acetaminophen and antipyrine were weak inhibitors of these enzymes. Structure-activity correlation suggests that an aromatic ring with a side chain containing a carboxylic acid is a requirement for competitive inhibition of the transformylase. The above listed NSAIDs also inhibited the folate-coenzyme-mediated biosynthesis of serine from glycine and formate (i.e., the C1 index) by human blood mononuclear cells (BMCs) in experiments where the drug was added to a culture of BMCs. Acetaminophen had a weak inhibitory effect on the C1 index. Consistent with the results obtained in votro is the observation that the C1 index of BMCs from rheumatoid-arthritis patients treated with drugs which possess little antifolate activity (e.g. acetaminophen) is higher than the C1 index of BMCs from rheumatoid-arthritis patients treated with NSAIDs possessing more potent antifolate activity (e.g. sulindac, sulphasalazine, naproxen and ibuprofen). The mean activity of the transformylase in BMCs taken from healthy humans was 1.98 nmol of product/h per 106 cells and the activity was positively correlated with BMC folate levels. These results are consistent with the hypothesis that (1) the antifolate activity of NSAIDs, and hence cytostatic consequences, are important factors in producing anti-inflammatory activity and (2) aspirin exerts its anti-inflammatory effects after its conversion into salicylic acid, which possesses greater antifolate activity than its parent compound.