Carboxyl group containing hydrazine-modified polyacrylonitrile (PAN) fibre was prepared by a modified method of T. Miyamatsu and N. Oguchi. The first investigation of this modified ion exchange fibre (IEF) revealed that the maximum capacities of the modified PAN fibre for the heavy metal ions Cu2+, Cd2+, Zn2+, Co2+, Pb2+, Cr3+, Ni2+ and Hg2+ were 1.33, 1.30, 1.03, 1.02, 0.98, 0.96, 0.95 and 0.63 mmol/g (dry weight), respectively. According to the distribution coefficients the order of decreasing selectivity of the cation exchanger for the various metal ions was Cu2+ > Zn2+ > Cr3+ > Hg2+ > Cd2+ > Pb2+ > Ni2+ > Co2+. The product could be reused after regeneration with diluted nitric acid. The high exchange capacity for copper, which offers possibilities for the application of the material for industrial purposes, was examined in additional experiments.