The results of comparative studies of the types of grain combine-harvesters gathering wheat for seed production are presented. One of them was a traditional grain combine-harvester equipped with one-drum threshing set with key shakers, powered continuously. The other one was a combine-harvester without shakers equipped with a rotary separator of grain from straw mass. In the study the influence of capacity, peripheral speed of drum and size of the slot between the drum and the threshing floor on biological quality of the output grain is presented. The research and the statistic analysis proved that grain gathered with the combine-harvester equipped with a multidrum separating set demonstrates considerably lower seed value in comparison with grain gathered with a combine of traditional construction.