To clarify the degradation pathway of acrinol by light, isolation and identification of acrinol degradation products (ANDP) were attempted. A novel acrinol degradation product, ANDP-8, one of the ANDPs, was isolated by extraction with methanol from cloths dampened with acrinol solution, and purified by column chromatography on Diaion HP-10 and Sephadex LH-20. The structural elucidation of ANDP-8 was examined by infrared, 1 H-NMR, 13 C-NMR and electron impact ionization (EI)-mass spectra studies. From the spectroscopic data, the structure of ANDP-8 was determined to be 2-ethoxyl-6, 9-acridinediamine-5, 8-dione, that was found to be a novel degradation product of acrinol by light. Antimicrobial activities of ANDP-8 against Gram-positive bacteria were 10 to 100-fold higher than those of acrinol. ANDP-8 was also active against yeast and fungi. Nevertheless, acrinol did not show growth inhibition even at a concentration of 100 g/ml.