The cellular location of lead and potassium has been studied in the lichen Cladonia rangiformis, collected from a lead-rich site. Elution of ions by dilute acid or their replacement by other ions has demonstrated that lead is ionically bound, most probably within the cell wall. The same techniques have shown a proportion of the total potassium to be also bound ionically in the cell wall. In addition potassium is present as freely diffusible material retained within the permeability barriers of the intact cell and also firmly bound in an insoluble form. The reported tolerance of lichens to heavy metals is discussed in relation to the demonstrated location of lead.