The auxin herbicide (4-chloro-2-methylphenoxy)-acetic acid (MCPA) was absorbed by liquid cultured callus cells of Phaseolus vulgaris L. and subsequently became hydroxylated at the methyl group. The alcohol remained largely unconjugated and was partly released to the culture medium. Several glycosides and an ether-soluble conjugate of MCPA appeared as minor metabolites. Unsubstituted phenoxyacetic acid (POA) was metabolised primarily by 4-hydroxylation and subsequent phenolic glucoside formation. Metabolites of POA were retained by the cells. In the case of both substrates, metabolism in cells correlated well with that observed in apical buds excised from etiolated seedlings.