A histological and immunohistochemical analysis of paraffin-embedded lymph nodes from 31 autopsies after bone marrow transplantation (BMT) was performed (20 allogeneic, 10 autologous, 1 syngeneic). Monoclonal antibodies against CD 45RB, CD 20, CD 21, CD 35. CD 43, CD 45RO, CD 76 and Ki-B3, and antisera for detection of S 100-protein and immunoglobulin isotypes were used. None of the lymph nodes showed a regular reconstitution. The lymphoid cells, scattered in a diffuse pattern, were mainly CD 43-positive. Most of them also expressed the CD 45RO antigen. CD 20- and Ki-B3 positive lymphoid cells were nearly absent within the first 100 days after BMT. After that time B cell follicles were detectable in a few cases. Surprisingly, in nearly all cases with infectious complications, numerous plasma cells could be found. The origin of this plasma cells is discussed.