Plants of the genus Allium such as onion and garlic are often consumed as a source of compounds which inhibit human platelet activity, with the goal of decreasing vascular disease. Antiplatelet activity in these plants is determined in part by native concentrations of organosulfur compounds. Evaluation of four onion genotypes grown in a field study at four US locations in 1994 demonstrated onions with mild flavor and low sulfur content exhibited significantly lower antiplatelet activity than those containing high levels of sulfur. Antiplatelet activity was significantly positively correlated with genotypically determined bulb sulfur content and dissolved solids, indicating these latter factors are good predictors of antiplatelet strength. These data demonstrate antiplatelet activity is genotype dependent and correlated with bulb sulfur content. Genotype and bulb sulfur content should be taken into account in studies assessing onion antiplatelet effects.