The present study concerns the immunocytochemical localization of S-100 protein and lysozyme in the cells of canine lymph nodes and lymphomas. In the normal canine lymph node S-100 protein was detected in follicular dendritic cells (FDCs) and in interdigitating reticulum cells (IRCs). S-100 positive cells were found in thirteen out of twenty-four lymphomas, scattered throughout the tumour or in follicles. In three cases (two cutaneous lymphomas and one nodal lymphoma) the number of positive cells was high, although the main proliferating cell population was S-100 negative. In normal lymph nodes lysozyme immunoreactivity was found in the cytoplasm of cortical and medullary histiocytes. Lymphomatous cells were negative for lysozyme, although in three cases lysozyme-positive cells were present between the lymphatic cells. The possible origin of these S-100 or lysozyme positive cells is discussed. It is suggested that S-100 protein and lysozyme can be used as useful markers for these two types of dendritic cells and for macrophages.