Apices of etiolated decapitated Alaska pea seedlings were painted with aqueous lanolin + IAA ± various inhibitors of RNA or protein synthesis. A subapical segment from the epicotyl was removed for measurements of growth and the soluble protein content and cellulase activity of enzyme extracts.During the first 18 hours, the main growth response to IAA was an increase in segment diameter; elongation was inhibited. The amount of extractable cellulase activity per segment and the diameter increased at exactly the same rates relative to controls. In the next 2 days IAA induced rapid cell division and the formation of root primordia. Cellulase activity per segment, per unit fresh weight, and per unit soluble protein all increased markedly to levels many times higher than in controls. Chloramphenicol, azaguanine, puromycin, and actinomycin D all interfered with protein synthesis, the growth responses, and the development of cellulase activity. In the absence of IAA, cellulase activity decreased.It is concluded that cellulase is subject to turnover in this tissue and that the rate of its synthesis is controlled by auxin concentration. It is proposed that cellulase action on microfibrils in vivo plays an essential role in a variety of growth processes, particularly lateral cell expansion.