In Jerusalem artichoke (Helianthus tuberosus L.) tuber tissue, cultured under optimal conditions (medium containing naphthaleneacetic acid (NAA). benzylamino purine (BAP), and soluble starch), 30–35% of all the cells differentiate into tracheids within 2 weeks. Under slightly different conditions (omission of BAP and starch from the optimal medium or addition of gibberellic acid (GA) or abscisic acid (ABA) to the optimal medium) the same growth rate is obtained with relatively few tracheids (3–5%).Activity of soluble and wall-bound peroxidase is higher in the presence of BAP, irrespective of the number of differentiating cells in the tissue. GA and ABA when added to the medium stimulate peroxidase activity while inhibiting differentiation by almost 90%. The variations in enzyme activity are related neither to the extent of differentiation nor to the rate of lignin biosynthesis in the tissue. Phenylalanine ammonia-lyase (PAL) (EC 220.127.116.11) activity shows some correlation with the amount of differentiation, but this activity is not in direct proportion to the number of differentiating cells under different conditions. Acid phosphatase (EC 18.104.22.168) activity does not change with time after 2 days of culture under any of the conditions. No qualitative differences have been observed in the isozyme patterns of these enzymes with time or in response to hormones.Incorporation of [14C]cinnamic acid and [14C]phenylalanine into cell wall lignin shows an increased rate of lignin biosynthesis in response to BAP. GA and ABA do not significantly affect lignin biosynthesis. Histochemical studies reveal no preferential lignification of differentiating tracheids. Lignin is distributed on parenchymatous as well as differentiated cells.