Colloidal carbon injected intravascularly provided a selective marker for post-capillary venules in lymph nodes and Peyer's patches. In the first few minutes a denticulate outline to the lumen was formed by carbon capping the high columnar epithelium and carbon particles were deeply embedded between endothelial cells. Ten minutes after injection carbon had reached the basement membrane and was lying outside this membrane 30 min later, at first free but later engulfed inside macrophages. Carbon was retained in post-capillary venules for the duration of the experiment (8 h). Discontinuities were present in the basement membrane in about one-fifth of venules, and lymphocyte and macrophage penetrating the basement membrane are demonstrated. It is postulated that carbon penetrates the post-capillary venular wall by increased intraluminal hydrostatic pressure arising from contraction of muscles that surrounded the lymphoid tissue in the case of the gut, or skeletal muscle compressing lymph nodes against a bony surface in the axilla, groin or neck. Secondly, if carbon is a model for particulate antigens, then post-capillary venules provide a potential immunogenic route whereby antigens can reach lymphoid tissues from the circulation, as proposed by Burwell (1962) for transplantation antigens.