The chemiluminescence of stimulated cells with phorbol myristate acetate and the production of nitric oxide after stimulation with lipopolisaccharide in the presence of the parent compounds FAA (flavone-8-acetic acid = (4-oxo-2- phenyl-4H-chromen-8-yl)acetic acid), XAA (xanthone-4-acetic acid = (9-oxo-9Hxanthen-4-yl)acetic acid), and appropriate xanthone derivatives (1-7) was determined. Also the toxicity of the FAA, MFAA ((6-methyl-4-oxo-2-aryl-4Hchromen-8-yl)acetic acid), XAA and 1–7 against J-774A.1 cultured cells was evaluated. Compound 5 (2-methyl-2-{[(9-oxo-9H-xanthen-2-yl)methyl]sulfanyl}- propanoic acid) was effective in inhibiting chemiluminescence of J-774A.1 cells but most of the other tested compounds stimulated the reaction. FAA and two xanthones with a methoxycarbonyl moeity slightly decreased the generation of nitric oxide at 50 μM. Most of the tested compounds (1-7) showed weak toxicity at concentration of 100 μM.