Replicalion-iocompetent retroviral vecton that encode the heritable marker enzyme, β-galactosidase, were used to study the lineage relationships of cells in the olfactory epithelium of unmanipulated animals and in the olfactory epithelium as it reconstitutes after lesion. Viralry-marked cells are categorized as to type based on their position in the epithelium and on expression of NCAM (limited to neurons) and the carbohydrate moiety recognized by Griffonia lectin (limited to the dark/horizontal basal cells and the microvillar class of supporting cells). Direct injections of the vectors into the olfactory epithelium of otherwise intact animals produce clusters of β-galactosidase-labeled cells when assessed 6–10 days after infection; these clusters are composed of neurons and NCAM- negative/lectin-negative light/globose basal cells exclusively. In contrast, clusters of virally-marked cells after MeBr-induced lesion of the epithelium frequently contain both neurons and supporting cells, M well as both types of basal cells. Other clusters contain supporting cells and/or Bowman's gland/duct cells. It is likely that the clusters of marked cells are derived from a single founder cell, i.e. the cells are clonal and lineally related, since the clusters are widely dispersed. Furthermore, infusion of mixtures of viruses that can be distinguished on the basis of die type and subcellular localization of the marker enzyme that is expressed produce clusters that are homogenous with respect to enzyme type, providing strong evidence in favor of the notion that the clusters are clonal in nature. Thus, the founders of the clones that contain neurons, supporting cells and basal cells are pluripotent in their capacity for differentiation. It is unlikely that the pluripotent cells are found in Bowman's gland/duct, since we have yet to observe a clone that contains neurons and cells in Bowman's gland/duct. Hence, the pluripotent stem cells are to be found in die basal cell compartment of the epithelium. However, the exact nature of these stem cells remains unknown and a subject for future investigation.