Abstract—As previously demonstrated, there is a considerable increase in the photodynamic activity of anthraquinone‐sulphonic acid on the addition of ethylene glycol, glycerol and sugars in concentrations of the order of 0.4M. This system exhibits cytostatic activity. With H2O2at higher concentrations there is also formation of aldehydes in approximately equivalent yields as follows:R.CH2OH+Q → QH2+R.CHOQH2+O2→ Q+H2O2In accordance with the recent results of Warburg this accounts for the fundamental importance of glyceraldehyde in cancer research.Our results on the irradiation of Proteus vulgaris SG2 in buffered medium with the addition of glycerin to anthraquinone‐sulphonic acid show the existence of a similar cytostatic effect. One hour's irradiation with visible light reduces the cell count by about 50