Macrophages, known as Hofbauer cells, are most abundant in placental villous stroma in the first and second trimesters. Their functions are not well defined. We have used a combination of in situ and in vitro methods to characterise these cells. Lectin histochemistry and immunohistochemistry were used to identify macrophages in situ. The lectin from Maclura pomifera (MPA) was found to mark cells bearing the CD68 antigen with optimal specificity and selectivity. MPA staining was used to show that they increase in number from mid first to mid second trimester, becoming much less abundant at term. The cells are absent from mesenchymal villi, being associated primarily with villous stroma containing the prominent interstitial channels characteristic of immature intermediate villi. A mixed stromal cell isolate was studied in monolayer culture, including the use of time-lapse microscopy. Cells from first or second trimester tissue contained a subpopulation of about 14-17% of cells that exhibited a macrophage-like morphology and expressed CD68 as well as MPA-binding glycans. These cells were short-lived in monoculture, but could persist in vitro in association with a fibroblast layer for several weeks. They could switch rapidly from a macrophage-like to a fibroblastic morphology, were highly motile and associated in clusters that rapidly formed and dissipated over periods of a few hours. These data suggest that Hofbauer cells play a role in the maturation of mesenchymal into immature intermediate-type stroma. They may be important in the excavation of stromal channels. Their prolonged viability in mixed cultures suggests a paracrine relationship with resident fibroblasts. Their location and migratory behaviour predict an ability to move rapidly around the villous stroma, perhaps within the channel system, and to make transient contacts both with other macrophages and stromal cells.