Sugar cane was grown in refined sand at different levels of Cu supply. Visible symptoms of Cu deficiency were observed at concentrations ? 0.0065 mg Cu litre-1 and Cu toxicity at 6.5 mg Cu litre-1. The fresh weight of millable canes and the concentration of sucrose in cane juice were highest at 0.65 mg Cu litre-1. Acute deficiency (0.00065 mg Cu litre-1) and toxicity (6.5 mg Cu litre-1) reduced the concentration of chlorophyll a and b, DNA, RNA, non-reducing sugars, and the activities of ascorbic acid oxidase, polyphenol oxidase, and cytochrome oxidase. The concentration of starch was reduced by acute deficiency and was increased by a toxic supply of copper. Very low supplies of Cu (? 0.0065 mg Cu litre-1) and excess supplies (6.5 mg Cu litre-1) increased the concentration of phosphorus, phenols, and activities of peroxidase, acid phosphatase, and ribonuclease. With an increase in Cu supply, the concentration of Cu in leaves and cane juice increased. Threshold of Cu deficiency could be indicated by the activities of ascorbic acid oxidase and cytochrome oxidase; the latter could also indicate the threshold of toxicity. Deficiency, threshold of deficiency, and toxicity values of Cu in young leaves at 35 weeks after sowing were, respectively, 4, 8.2, 19 ?g g-1 dry matter.