The toxicity of several toluidine and thiocarbamate herbicides to johnsongrass [Sorghum halepense (L.) Pers.] rhizome tissue was determined by a triphenyl tetrazolium chloride (TTC) assay. Rhizome sections were incubated in herbicide solutions for 24 hr and then in TTC solution for 24 hr. The amount of reduced TTC was a quantitative measure of herbicide toxicity. Concentrations of the different herbicides required to cause 100% tissue injury indicated that the order of decreasing herbicide toxicity was: USB-3584 (N3,N3-diethyl-2,4-dinitro-6-trifluoromethyl-1,3-phenylendiamine); trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine); benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2-6-dinitro-p-toluidine); nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline]; San-9789 [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazione]; and A-820 (Nsec-butyl-4-tert-butyl-2,6-dinitroaniline). In another study CGA-11607 (N-ethyl-N-tetrahydrofurfuryl-4-trifluoromethyl-2,6-dinitroaniline); AN-5647 [N,N-bis(2-chloroethyl)-2,6-dinitro-p-toluidine]; and CGA-10832 (Nn-propyl-N-cyclopropylmethyl-4-trifluoromethyl-2,6-dinitroaniline) were less toxic than trifluralin to johnsongrass rhizomes. Complete tissue injury caused by the thiocarbamates occurred in the same concentration range (10−3 to 10−2M) as the toluidine herbicides. At a concentration required for complete kill the toxicity in decreasing order was: EPTC (S-ethyl dipropylthiocarbamate); vernolate (S-propyl dipropylthiocarbamate); butylate (S-ethyl diisobutylthiocarbamate); and pebulate (S-propyl butylethylthiocarbamate). R-25788 (N,N-diallyl-2,2-dichloroacetamide) at 6.7 × 10−5 M had no effect on vernolate, EPTC, or pebulate activity; but it increased the toxicity of butylate to johnsongrass rhizome tissue.