A laboratory assay method was developed for studying pricklypear (Opuntia sp.) responses to herbicides. Herbicidal translocation from one pad to another and lateral translocation across a pad appeared to be very slight. Pads were killed when a butoxy ethanol ester, a propylene glycol butyl ether ester, or a 2-ethylhexyl ester formulation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) was applied to only one side. Increasing temperatures from 21 to 43 C increased the rate of killing pads. Concentrations of 2,4,5-T at 8 lb aehg of solution usually killed the pad at temperatures above 21 C. Concentrations of 2,4,5-T at 4 lb aehg or less had little effect on the pads. Relative humidity level had little effect on 2,4,5-T activity, but the activity of a propylene glycol butyl ether ester of 2,4-dichlorophenoxyacetic acid (2,4-D) was more effective at the 90 to 95% level than at levels ranging from 30 to 75%. Water, diesel oil, and three diesel oil:water mixtures had little effect on 2,4,5-T toxicity. More toxic effects were shown by 2,4,5-T than 2,4-D. The propylene glycol butyl ether ester, the butoxy ether ester, and the 2-ethylhexyl ester of 2,4,5-T were equally effective on pricklypear at the same concentrations.