The superficial inguinal lymph nodes of 10 piglets which had died spontaneously of post-weaning multisystemic wasting syndrome (PMWS), in which the porcine circovirus type II (PCV-II) genome was revealed by PCR, were submitted to immunohistochemical investigation for CD4, CD8, IgM, MAC387, S-100 protein, vimentin and F-VIII-RA and compared with three normal cases. The lymph node reaction was graded as initial, intermediate and end stage according to histological criteria. In the initial and intermediate stages, absence of follicles and depletion of lymphocytes were evident. Associated with this was a reduction in numbers of interfollicular dendritic cells and interdigitating cells and a reduction/absence of B cells and mainly CD4+ T lymphocytes. In the end stage the reduced expression of high endothelial venules and the prevalence of the stromal component of the lymph node was prominent, as well as the above changes. It is concluded that more than one mechanism is involved in the immunosuppressive ability of PCV-II: reduction of the antigen presenting ability and reduction of B cells and CD4+ T cell function.