The effect of the size and shape of drug active substance particles on wet granulation has been studied. Active substances delivered by two different producers and active substance sampled from upper and lower levels of a storage container were used in pilot-plant experiments. There were marked differences in particle size and shape according to bulk and tapped densities, sieve and microscopy analyses. Consequently, marked fluctuations in granulation period, impeller power consumption and granulated mixture temperature increases were detected by the pilot-plant experiments with investigated active substances.