Abstract Plants have been reported not to metabolize phosphinothricin. In order to reexamine these literature findings, cell suspension cultures of soybean ( Glycine max L.), wheat ( Triticum aestivum L.), and maize ( Zea mays L.) were incubated with l-[3,4- 14C]phosphinothricin. In maize cells which took up up to 50% of the applied radioactivity four different metabolites were detected. They were identified as 4-methylphosphinico-2-oxo-butyric acid, 4-methylphosphinico-2-hydroxybutyric acid, 4-methylphosphinico-butyric acid, and 3-methylphosphinico-propionic acid, respectively. A new HPLC separation method was developed in order to identify these unusually polar metabolites. In soybean and wheat cultures, 10 and 6%, respectively, of the applied radioactivity was taken up. In soybean only one metabolite, 3-methylphosphinico-propionic acid, could be detected whereas in wheat 4-methyl-phosphinico-butyric acid was additionally present. Radioactivity in the culture media could be attributed to unchanged herbicide in all cases. 14CO 2-evolution from soybean and maize cultures was very low (<0.1% of the applied radioactivity within 4 days). Incorporation of radioactivity from applied [ 14C]phosphinothricin into bound residues also reached only small amounts (<0.1%). A metabolism scheme for phosphinothricin is proposed.