A series of experimental letterpress printing runs has been performed of the Graphic Arts Research Laboratory in Sweden and the effect on the filling up of shadows in halftone plates of the following variables was investigated: number of sheets printed, screen ruling and etching depth of the plates, amount of ink on the plates, ink viscosity, printing pressure, type of inking rollers and setting of inking rollers. Printing press, printing speed and paper were the same during all runs and temperature as well as relative humidity were strictly controlled. These experiments which, however, may not be looked upon as concluded have shown that the amount and viscosity of the ink have an appreciable effect on the filling up of the shadows whereas the influence of the printing pressure and the inking roller setting is remarkably little. Inking rollers of various material caused quite different amount of filling up, but it has not yet been studied, which properties of the rollers cause this difference. Relatively small differences in etching depth had under certain circumstances a marked effect on the amount of filling up. It was evident that most of the variables studied have no main effects, but apparently rather complicated interactions exist between them.