K+ efflux from Umbilicaria muhlenbergii was not affected following the uptake of known amounts (< 20 μmol g-1) of Ca2+, Mg2+, Sr2+ and Zn2+, but was increased in the cases of Cu2+ and Pb2+. In a number of samples that had taken up increasing amounts of Cu2+, a discontinuity in K+ release during metal incubation correlated with a decrease in subsequent 14C fixation. Some of the binding sites associated with the first phase of Cu2+ uptake (up to 12 μmol g-1) are interpreted to occur near, on or within the algal cells. The larger K+ loss associated with higher Cu2+ uptake levels indicated that a component of the second Cu2+ uptake phase involved binding at or penetration of the fungal membranes. The uptake of Sr2+, Ni2+ and Zn2+ conferred some protection to samples subsequently exposed to 75 p.p.m. aqueous SO2 for 1 h, while Mg2+, and Ca2+ had no effect. The combined effects of Cu2+ uptake and SOS exposure (and also Pb2+ and SO8) were approximately cumulative. Finally, the results are explained by reference to a classification that separates metal ions into three chemically and biologically significant categories. It is concluded that class A metal ions (those with a preference for ionic interactions) and borderline metal ions with class A character tend to protect lichens against SO2 damage, while borderline metal ions with class B character (a preference for covalent interactions) have the opposite effect.