We have recently described the existence of two subsets of porcine monocytes based on the expression of CD163. In this study we compare the expression of a number of cell surface antigens in CD163+ and CD163- monocyte subsets using three-color flow cytometry. These monocyte subsets show differences with respect to the expression of MHC class II antigens (SLA-DR and DQ) and a variety of adhesion molecules (CD11a, wCD11c, wCD29, CD49d) that are expressed at higher levels on CD163+ monocytes, and of CD14 that is higher expressed by CD 163- cells. These differences on phenotype could reflect differences in the ability of these two subsets to migrate to tissues and may account for the higher allostimulatory capacity of CD163+ cells. In some aspects, the phenotype of CD163+ monocytes resembles that of mature macrophages. In vitro serum-induced maturation of monocytes into macrophages lead to the expression of SWC9 together with an increase in the expression of CD163 and a reduction in that of CD14. These results delineate a maturation pathway where CD14hiCD163-SWC9- monocytes develop into CD14loCD163+SWC9- monocytes and these cells into CD14loCD163+SWC9+ macrophages.