Changes in metal-resistance following pre-exposure to sub-lethal concentrations of Zn or Ni are reported in sycamore (Acer pseudoplatanus L.) callus cell lines established from shoot explants of mature trees. A cell line derived from a site contaminated primarily by Cu, Cd and Zn possessed a higher resistance to Zn and Ni than did a cell line from an uncontaminated site. Repeated subculturing of both cell lines on 30 mg Zn l - 1 and 5 mg Ni l - 1 led to respective increases in Zn- and Ni-resistance. Resistance could be induced within an optimum range of media metal concentrations, and there was a clear limit to the level of metal-resistance achievable by induction. There was some evidence of co-resistance between Zn and Ni; selection on 5 mg Ni l - 1 did not increase Zn resistance of either cell line but repeated subculturing on 30 mg Zn l - 1 considerably enhanced Ni resistance at critical test concentrations.