The p24 protein is the major core protein of LAV/HTLV III which is the putative agent of the lymphadenopathy syndrome. By the use of an anti‐p24 monoclonal antibody we have studied the presence of LAV/HTLV III infected cells in 20 lymph nodes obtained from lymphadenopathy syndrome patients: 14 lymph nodes were characterized by prominent follicular hyperplasia consistent with the early phase of the syndrome and six lymph nodes presented marked regressive changes. Cells positive for p24 were detected in 8/14 lymph nodes with hyperplastic changes and in 1/6 lymph nodes with regressive changes. Positive cells were most often located in germinal centres and were mainly characterized by a lymphoid morphology. However, immunoreactivity for p24 protein was also occasionally observed in some histiocytic‐like cells and in high endothelial cells of post‐capillary venules, suggesting that these ‘accessory cells’ also play a role in the early phases of the lymphadenopathy