In the present study, we examined the expression and distribution of both hyaluronan and its cell-surface receptor (CD44) during lung development in the mouse. Hyaluronan was detected by a specific binding probe, termed b-PG, which is a biotinylated mixture of proteoglycan and link protein from cartilage. Using this probe in an enzyme-linked assay, the amount of hyaluronan in relation to protein content was found to decrease as lung development progressed. In addition, histochemical staining of the embryonic lungs revealed that during early stages, relatively large amounts of hyaluronan were present in the interstitium. However, as development progressed, much of this was lost, and in the adult, hyaluronan was restricted to the regions surrounding the major blood vessels, bronchi, and bronchioles. In contrast to hyaluronan, the amount of CD44 increased as a function of development, as determined by the rat monoclonal antibody, KM-201. Histochemical staining with this antibody showed that the receptor was primarily associated with macrophages and to a lesser extent with adult bronchial and bronchiolar epithelium, vascular smooth muscle, and endothelial cells. As development progressed, the macrophages expressing CD44 increased in number, and this increase was temporarily correlated with the decrease in hyaluronan content. In addition, histochemical staining revealed that some of these macrophages contained hyaluronan in their cytoplasm, suggesting that macrophages had internalized hyaluronan from the extracellular matrix. This possibility was further supported by the fact that when newborn mice were injected with the KM-201 monoclonal antibody, which blocks the interaction between hyaluronan and the receptor, the number of hyaluronan-containing macrophages in the lungs decreased while the concentration of hyaluronan increased. Taken together, these results suggest that macrophages can internalize hyaluronan during lung development and could possibly play a significant role in its removal.