Using an indirect immunofluorescent technique, expression of cell surface fibronectin and a cell surface fibronectin-binding molecule was studied during mouse embryo preimplantation development. We also studied the expression of fibronectin on immunosurgically isolated inner cell masses (ICMs) and regenerated mouse blastocysts. Fibronectin and the fibronectin-binding molecule were not detected at the morula stage. From the early to late blastocyst stage, fibronectin expression increased on the trophectoderm. Expression of the fibronectin-binding molecule was found only in the polar trophectoderm region of the early blastocyst, then in the polar and mural trophectoderm regions of the middle blastocyst. In the late blastocyst stage, this fibronectin-binding molecule was only present in the mural trophectoderm. Fibronectin expression by ICMs of early blastocysts was more intense than that of late blastocysts. After 24 h of culture, 10% of ICMs isolated from early blastocysts regenerated a trophectoderm which stained intensively for fibronectin in the mural and polar trophectoderm regions. After 48 h of culture, regenerated blastocyst-like structures closely resembled the normally obtained late blastocysts and stained for fibronectin in the mural and polar trophectoderm regions. The significance of the results is discussed in relation to mouse embryo development, trophectoderm formation and blastocyst implantation.