A rapid analytical screening by high-performance liquid chromatography combined with enzyme-linked immunosorbent assay specific for N6-(meta-hydroxybenzyl)adenine and its 9-substituted derivatives revealed several regions of immunoactivity in extracts of mature poplar leaves (Populus x canadensis Moench., cv. Robusta). The identity of the cross-reactive compounds was investigated by UV and mass spectrometry and after permethylation of unknown substances by gas chromatography-mass spectrometry. Physico-chemical analysis indicated the putative cytokinins are N6-(meta-hydroxybenzyl)adenine and its 9-β-D-ribofuranosyl or 9-β-D-glucopyranosyl derivatives. The chemical structure was subsequently confirmed by synthesis. For N6-(meta-hydroxybenzyl)adenine, a highly active aromatic cytokinin, we propose the trivial name 'meta-topolin'.