When incorporated into fixatives, lanthanum was preferentially localized to nuclear pores in a variety of cells. Pore complexes other than nuclear pores did not stain. This preferential staining occurred with a concentration as low as 5 mM. When cells were prefixed with aldehyde and subsequently exposed to lanthanum, the staining pattern was inconsistent suggesting that the tracer did not permeate the cell membrane in adequate concentration to reach the nuclear-pore complexes. Although the reason for this affinity of the tracer to nuclear pores is not clear, lanthanum can be used as a probe for nuclear pore complexes.