Autoradiographic in vitro-examination of normal and of psoriatic skin, using the double labelling technique (1 µC thymidine-2-C14 and 10 µC thymidine-methyl-H3), revealed the usual increase of proliferative activity in psoriatic skin, approximately four times greater than normal. The cell cycle time was shortened. According to the data obtained by the double labelling technique a shortening of the DNA synthesis time was not found in vitro. Treatment with a dithranol-salicylic acid (10%)-petrolatum ointment (DSP) normalized the cell cycle in accordance with the degree of clinical improvement. The findings seem to indicate that DSP has an effect on the DNA-synthesis.