Expression of CD43 (leukosialin, sialophorin) by rat blood monocytes was analyzed by flow cytometric, microscopic, and biochemical techniques. Monocytes were identified cytometrically using a combination of light-scatter parameters and binding of the anti-monocyte/macrophage monoclonal antibody (mAb) OX-41. Two-color flow cytometry studies with W3/13 and HIS 17, two anti-CD43 mAb that react with different antigenic epitopes, revealed two subpopulations of monocytes expressing disparate levels of CD43 (referred to hereafter as hi-CD43 MO and lo-CD43 MO). In three-color flow cytometry studies, hi-CD43 MO were found to express higher levels of CD4 than lo-CD43 MO; in contrast, lo-CD43 MO bound higher levels of RP-3, a mAb raised against rat neutrophils. Hi- and lo-CD43 MO expressed comparable amounts of CD14, CD45, and intercellular adhesion molecule-1 (CD54); hi-CD43 MO expressed somewhat more lymphocyte function-associated antigen-1 alpha chain (CD11a) and CD18 than their lo-CD43 MO counterparts. A minority of cells in both subpopulations expressed class II histocompatibility antigens. Hi- and lo-CD43 MO isolated by fluorescence-activated cell sorting had features typical of monocytes as assessed by light and electron microscopy. In Western blotting experiments, lo-CD43 MO and elicited peritoneal macrophages were found to express a less heavily sialylated form of CD43 than hi-CD43 MO.