Hyaluronan was localized in postimplantation mouse embryos using CD44, the principal hyaluronan receptor. The specificity of CD44 receptor-globulin labelling was confirmed using Streptomyces hyaluronidase, anti-chondroitin sulfate antibody, and other receptor globulins. Our major findings are summarized as follows: 1. Implantation of the blastocyst into the uterine wall triggers a rapid loss of hyaluronan from the extracellular matrix of decidual cells on the anti-mesometrial side of the uterus. 2. Hyaluronan appears early in development in the yolk cavity, and the basement membranes of primitive ectoderm and primitive endoderm. 3. During gastrulation, mesodermal cells enter a hyaluronan-rich environment, but lack a pericellular hyaluronan coat themselves. 4. In limb bud embryos, hyaluronan is present throughout the cranial mesenchyme, but is generally not present in the branchial bars, somites, or limb buds. 5. At mid-gestation, hyaluronan is present in the axial skeleton, craniofacial mesenchyme, endocardial cushions of the heart, smooth muscle of the gastrointestinal tract, and connective tissue throughout the body. The pattern of hyaluronan expression in the day 13 fetus is nearly identical to the published distribution of transforming growth factor beta (TGF beta), suggesting a close functional relationship between these molecules. Together, the results suggest that hyaluronan is involved in the formation of early mesoderm, differentiation of craniofacial mesenchyme, and morphogenesis of the axial skeleton