A monoclonal antibody, 6D2, is described that recognizes a different epitope on the NLDC-145 dendritic cell associated molecule in the mouse. During ontogeny the epitope appears on interdigitating cells in lymphoid organs only around birth, whereas the NLDC-145 antigen can be detected as early as day 16 of gestation. No differences can be observed in the expression of the two antigenic determinants on the epithelial cells of the thymus during ontogeny. Evidence is presented that the two antibodies recognize different epitopes on the same molecule.