Rabbit T lymphocytes were characterized using fluorochrome-labelled antisera against thymus cell determinants and fluorochrome-labelled lectins. Three anti-T cell antisera were used, detecting different antigenic determinants on the majority of T cells. Only small subpopulations were found stained by one of the three antisera. After dexamethasone (DX) treatment, the proportion of T cells was significantly increased in bone marrow and appendix, presumably by different mechanisms. Cells binding peanut agglutinin, mainly belonging to the T cell population, were present in small numbers in thymus, spleen, lymph node, and appendix from normal as well as from DX-treated animals. In bone marrow, however, a large PNA+ population showing neither T nor B cell surface properties was observed. After treatment with neuraminidase (NA), PNA binding sites were exposed on B as well as on an increased number of T cells. It is suggested, therefore, that T and B cells retained their PNA receptors during maturation. Masking of these receptors will take place before differentiation of T cells is initiated within the thymus. After NA treatment, also binding sites for Helix pomatia agglutinin were exposed on T cells and to different extents, revealing subsets of negative, weakly and strongly positive HPA cells. HPA weakly positive cells form the major population present in the thymus, while HPA strongly positive cells constitute the major population of the T cells in spleen and lymph node. The exposure of receptors for PNA of HPA appears not to be related to the steroid sensitivity of the cells.