In tests conducted at Iowa country elevators, mechanical grain probes were compared with the official hand probe. Comparisons were based on the percentage of foreign material (FM) in corn and soybean samples. In-load suction probes over-estimated FM by about 1.50 points in corn and 1.05 points in soybeans. Performance of gravity-fill and core probes varied among models and between grains. The core probe was the most accurate design for FM sampling. In a laboratory experiment, the official hand probe collected corn samples with higher BCFM content than known control concentrations. At 3% BCFM, the hand probe collected 3.5%. Hand probe errors increased with the square of BCFM content.