The addition of simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] wettable powder to water solutions of glyphosate [N-(phosphonomethyl)glycine] in greenhouse studies reduced glyphosate activity on quackgrass [Agropyron repens (L.) Beavu.], corn (Zea mays L.), and beans (Phaseolus vulgaris L.). Atrazine [2-chloro-4-(ethylamino)-6-isopropylamino)-s-triazine] wettable powder reduced glyphosate activity on corn. This antagonism was more pronounced at threshold levels of glyphosate and was overcome by increasing the glyphosate rate. Simazine applied to corn through the soil did not reduce the activity of glyphosate applied to the foliage. Glyphosate activity on corn was reduced equally when glyphosate was mixed with 4.2 kg/ha of commercial simazine wettable powder or an equal amount of the inert ingredients used in the commercial product. When mixtures of glyphosate with either the commercial simazine product or the inert ingredients from the commercial product were centrifuged, the amount of phosphorus (used as a measure of glyphosate) in the supernatant was reduced by 10 to 20%. When the supernatants of the various mixtures were applied to bean plants, the reduced phytotoxicity from the mixtures was comparable to the antagonism noted in previous experiments. These data support the conclusion that the reduction in glyphosate phytotoxicity from mixing with atrazine or simazine wettable powders is due to a physical binding within the spray solution rather than from biological interactions within the plant.