Field studies of herbicidal antagonism were conducted in corn (Zea mays L.), soybeans (Glycine max L. Merr.), and on industrial sites. The addition of chlorbromuron [3-(4-bromo-3-chlorophenyl)-1-methoxy-1-methylurea], cyanazine {2-[[4-(chloro-6-(ethylamino)-s-triazin-2-yl] amino]-2-methylpropionitrile}, bifenox [methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate], or atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] wettable powder plus dicamba (3,6-dichloro-o-anisic acid) to glyphosate [N-(phosphonomethyl)glycine]-alachlor[2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] combinations reduced the activity of glyphosate on quackgrass [Agropyron repens (L.) Beauv.], common dandelion (Taraxacum officinale Weber), and Canada thistle [Cirsium arvense (L.) Scop.] in no-till corn. Reduced weed control from antagonism resulted in decreased corn yields. Linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea], chlorbromuron, or metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], when added to glyphosate, reduced the control of quackgrass but not that of Canada thistle, alfalfa, or common dandelion in soybeans. Antagonsim was not evident in annual weed species. In perennial weeds, the degree of antagonism was often reduced with increased dosages of glyphosate. The inclusion of terbacil (3-tert-butyl-5-chloro-6-methyluracil), bromacil (5-bromo-3-sec-butyl-6-methyluracil), and simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] with glyphosate in mixes interfered with the control of smooth brome (Bromus inermis Leyss.), but not when applications of the residual herbicides were delayed. Quackgrass control was reduced when amitrole (3-amino-s-triazole) was mixed with glyphosate, or when applied separately.