Abstract The uptake and metabolism of the herbicide triclopyr (3,5,6-trichloro-2-pyridinyloxyacetic acid) by cell suspension cultures of soybean, Glycine max. var. Harcor were investigated in comparison with those of 2,4-D. After 7 days two major metabolites of triclopyr were formed, in ca. 1.5:1 ratio. These were isolated by ion-exchange and C 18-high-performance liquid chromatography. Combined gas chromatography-mass spectrometry of their methyl esters identified the metabolites as the aspartate (major) and glutamate (minor) amide conjugates. In contrast, 2,4-D was predominantly metabolized to two more polar products having the properties of glucosyl ethers of hydroxylated 2,4-D. The effect of both incubation period and growth medium upon the behavior of soybean cells with respect to these herbicides was also studied. Cells grown in Miller's medium supplemented with naphthyl-1-acetic acid took up slightly more [ 14 C] triclopyr , but significantly more [ 14 C]2,4- D , than those grown in Gamborg's B5 medium supplemented with 2,4-D. In either medium, active metabolism of absorbed herbicides continued until at least 21 days, with the qualitative nature of products being only substrate dependent.