sufficient time and at sufficient temperature to hydrolyze the ester linkages such that there is less than 10 preferably less than 4 5 esters in the composition in one embodiment the acetylated hematoporphyrin is reacted with 0 1n sodium hydroxide for at least an hour at room temperature the solution is purified using room temperature diafiltration and stored at 4 u00b0 c from 14 to 21 days the material is then repurified using a diafiltration procedure in an alternate procedure the acetylated hematoporphyrin is dissolved in either 0 1n or 0 2n sodium hydroxide and allowed to react for 16 to 24 hours at room temperature the solution is then purified using a room temperature diafiltration procedure or using a diafiltration procedure at 35 u00b0 40 u00b0 c using an additional method the acetylated hematoporphyrin is dissolved in either 0 1n or 0 2n sodium hydroxide and allowed to react for one hour at 35 u00b0 45 u00b0 c the solution is then purified at room temperature or at 35 u00b0 45 u00b0 c using a diafiltration procedure alternatively the acetylated hematoporphyrin is dissolved in 0 1n sodium hydroxide and allowed to react for one hour at room temperature additional sodium hydroxide is then added to a concentration of 0 2n and held there for 10 24 hours or a 1n concentration and held for 2 hours the solution is then purified at room temperature using the diafiltration.