Greenhouse and laboratory experiments were conducted to investigate the response of common lambsquarters to POST applications of halosulfuron–methyl plus 2,4-D admixtures and to investigate the effects of 2,4-D on the absorption, translocation, and metabolism of halosulfuron. In the greenhouse, halosulfuron at 0, 4.5, 9, 18, and 36 g ai/ha was applied alone and mixed with 2,4-D at 0, 17, 35, and 70 g ai/ha POST to 7.5- to 9-cm seedlings, and plant fresh weights were determined 4 wk after treatment (WAT). Halosulfuron alone did not control this weed, while fresh weights of common lambsquarters treated with 2,4-D declined hyperbolically as rates increased. A synergistic response for mixtures of these herbicides occurred, as observed fresh weights for all combinations were less than expected based on independent action and the calibrated marginal responses. In the laboratory, 7.5- to 9-cm seedlings were treated POST with commercially formulated halosulfuron at 9 and 18 g/ha and 2,4-D at 0, 70, and 140 g/ha, respectively, followed by foliar-applied 14C-halosulfuron. Absorption of 14C-halosulfuron increased with time, and absorption and translocation were not influenced by the addition of 2,4-D. Results from these studies inferred that halosulfuron and 2,4-D were generally synergistic on common lambsquarters and that mechanisms other than absorption, translocation, and metabolism may explain this response.