Stripper harvesting differs from conventional cereal harvesting methods by removing only grain, leaf and a proportion of straw, leaving most straw in place. A horizontal stripping rotor carrying eight banks of comb-like stripping elements rotates at approximately 650rev/min, speed being adjusted to suit different crop conditions. Contact between the stripping elements and cereal heads can result in the detachment of the whole head or its threshing in situ. The proportion of crop threshed in this way varies in different crop conditions but typically in wheat it is around 80%. Therefore, the possibility exists of the early separation of this threshed grain, i.e. ahead of the threshing drum into which the remainder of the crop must pass. Possible benefits include prevention of damage to the "pre-separated" grain and reduced grain losses from the threshing and separation mechanisms because of reduced loading.One simple design of mechanical separator, using a perforated screen below a conventional crop auger, was evaluated. Pre-separator efficiency was closely related to the type and condition of crop because this affected the straw intake and the percentage of grain threshed at the point of stripping. Grain removal ranged from less than 20% to more than 80% of total. Removal of material other than grain (MOG) ranged from less than 1% to 25% representing chaff only. Pre-separated grain included "whitecaps" or awned grains in some crops. Design details such as screen aperture size, shape and percentage open area were varied and had minor effects compared to crop factors outlined.The separation efficiency of a standard threshing drum and concave was found to be altered little by the use of the pre-separator in field experiments and in laboratory experiments simulating field conditions. It was determined by the rate of MOG throughput and was almost independent of grain throughput. While drum separation efficiency remained largely the same, the effect of using the pre-separator was to reduce the amount of grain available for separation by the drum. Where the pre-separator removed, for example, half of the total grain, the threshing drum handled only the other half, so running at the same separation efficiency, its "loss" was halved.