Anti-rat CD11b/c monoclonal antibody recognizes most macrophages including resident peritoneal macrophages, kupffer cells, but only approximately 35% of alveolar macrophages (2). ABIN133349also labels dendritic cells extensively, granulocytes and cells with morphology of microglia in the brain. It precipitates three polypeptides of MW 160, 103 and 95 kDa. ABIN133349inhibits complement mediated rosettes and is probably the rat equivalent of the human receptor for C3b called CR3. anti-rat CD11b/c monoclonal antibody should prove useful in the recognition of macrophages and microglia.