Emerging identity-preserved grain markets depend on avoidance of commingling grain at harvest. Knowledge of where grain resides in a combine, cleaning labor requirements, and resulting purity levels would assist producers. Measurements were made of grain and other material residing in different areas of rotary- and cylinder-type combines in replicated clean-outs during corn and soybean harvest and also in preliminary clean-outs during oat harvest. Concentration of the prior (i.e., commingled) grain was measured in the first grain harvested of the subsequent crop.