The invention relates to a method for controlling the crystal size during continuous mass crystallisation, especially with ammonium sulphate. According to said method, seed products are added, the seed product being produced independently of the current crystallisation process in terms of its parameters. The average grain size of the solid form of the seed product is 0.1 to 1.0 mm and is smaller than that of the desired crstallisate. The solid form of the seed product is produced from different technological sub-streams in the given grain size range, independently of the main crystallisation process. The temperature of the seed product when it is added is up to 40 °C, preferably 10 to 30 °C lower than the process temperature in the crystalliser and all other materials that are supplied or returned to the crystalliser are free of solids. By controlling the parameters of the seed product it is possible to influence the grain size distribution of the end product and significantly reduce the fluctuations in the distribution of the grain size of the end product. The method can be carried out with discontinuous or continuous addition of the seed product.