Expression of Ia-antigen on cells in lymphoid dendritic cells (LDCs)-enrt- ched fractions isolated from unstimulated rat spleens was observed with ABC immunoperoxidase and indirect immunofluorescent staining methods. The results appeared that there were three types of cells in stained preparaions: 1. Large cells with dendritic processes on their surfaces showed dark brown in colour with ABC staining method and emitted intense light-green fluorescence with immuno- fluorescent staining technic. 2. Large rounded or ovoid cells without obviously ramifications revealed colourless or light yellow-brown in colour with ABC sta- ining method and emitted no or weak yellow-green fluorescence with immuno- fluorescent staining technic. 3. Spherical lymphocytes of varying in size were similar to type 2 cells in immunoreactivity. The results of SEM and S-100 pro- tein staining confirmed that these three types of cells were LDCs, macrophages and lymphocytes in turn. It is indicated that LDCs consistently expressed Ia- antigen in large amount on their surfaces.