Summary The additon of activated charcoal (AC) to several tissue culture systems has a promotory effect on the growth of the tissue. In an attempt to explain this effect, some of the properties of AC have been investigated, and it has been shown to adsorb auxin and cytokinin from culture media, thus rendering them inactive. Auxin, as α -naphthylacetic acid can be adsorbed in concentrations up to 300 mg l −1 . Three cytokinins, 6-furfuryl-aminopurine, 6-benzylaminopurine, and 6-( γ , γ -dimethylallylamino) purine were adsorbed equally at a concentration of 10 mg l −1 . AC which had been used in tissue culture media was examined by 1H nuclear magnetic resonance spectroscopy and found to contain 5-hydroxymethylfurfural (HMF). HMF is produced by sucrose dehydration during autoclaving, and in the absence of AC will inhibit the growth of Nicotiana tabacum anther cultures.