Outbreaks of contagious bovine pleuropneumonia (CBPP) were reported in Lombardy, Northern Italy, at the end of 1990. For the purpose of this study, 54 slaughtered Holstein-Friesian cows showing typical lung lesions of CBPP from which the small colony type of Mycoplasma mycoides subspecies mycoides (M. m. mycoides SC) was isolated, were selected. Thoracic lymph nodes from these animals were sampled for bacteriological, histological and immunohistochemical analysis. Acute, subacute and chronic lesions were observed in 13, 12 and 29 cases, respectively. In the 13 animals showing acute lung lesions, an increased number of macrophages was observed, especially in the subcapsular sinuses, but frequently also in the cortical and medullary sinuses of the thoracic lymph nodes; in all 13 acute cases M. m. mycoides SC antigen was detected immunohistochemically in the cytoplasm of the macrophages. In 10 out of the 12 cases with subacute lung lesions, mycoplasma antigen was observed in macrophages located in sinuses, as well as in those scattered in the lymph node parenchyma. Hyperplasia of germinal centres in follicles was observed histologically in most of the 29 cases with chronic lung lesions. In immunohistochemically labelled sections, the characteristic finding observed in 27 of the chronic cases, was the presence of a variable amount of positive material in the germinal centres. These findings demonstrate the involvement of thoracic lymph nodes in CBPP.