Fibrotic processes in tissues are characterised by an excessive accumulation of collagen containing increased levels of pyridinoline cross-links,which has been taken as an important criterion to diagnose the fibrosis.Hypocrellin B(HB) is a natural photosensitizer with excellent photodynamic activity but very low dark-toxicity.In the current work,main purpose was to clarify if photodynamic action of HB could be applied to regulating the level of pyridinoline cross-link in collagen.To use gelatin as a model collagen,it was observed that the pyridinoline cross-link formation was effectively degraded by HB photodynamic action in the gelatin,suggesting the level of pyridinoline cross-link formation in collagen may be photodynamically regulated to a normal level.Furthermore,mechanism of the photodynamic action of HB was studied. Fibrotic processes in tissues are characterised by an excessive accumulation of collagen containing increased levels of pyridinoline cross-links, which has taken as an important criterion to diagnose the fibrosis. Hypocrellin B (HB) is a natural photosensitizer with excellent photodynamic activity but very low dark-toxicity.In the current work, main purpose was to clarify if photodynamic action of HB could be applied to regulate the level of pyridinoline cross-link in collagen.To use gelatin as a model collagen, it was observed that the pyridinoline cross- link formation was effectively degraded by HB photodynamic action in the gelatin, suggesting the level of pyridinoline cross-link formation in collagen may be photodynamically regulated to a normal level. Stillrther, mechanism of the photodynamic action of HB was studied.