Small nuclear ribonucleoproteins (snRNPs) were localized using human autoimmune or monoclonal anti-snRNP antibodies and ultrastructural immunocytochemistry in early preimplantation bovine embryos before and at transcription onset. Bovine cleavage stages up to 16-cell embryos were obtained either by culture of in vitro-fertilized ovarian oocytes or by isolation at slaughter of embryos fertilized and developed in vivo. Before transcription onset, up to the four-cell stage, diffuse labeling of nucleoplasm was detected, whereas cytoplasm labeling remained low. At the transcription onset, labeling of all eight-cell embryo nuclei was markedly concentrated at the borderline of already formed, condensed chromatin aggregates, where it was associated mainly with perichromatin fibrils. The condensed chromatin blocks revealed by several different staining methods were more prominent than is the case in most somatic cells. The degree of chromatin condensation and the pattern of its distribution differed between in vivo- and in vitro-produced eight-cell embryos: the in vitro embryos showed a higher degree of chromatin condensation and a peripheral distribution of chromatin blocks. A relation of this observation to the developmental potential of cow embryos is suggested. In two- and four-cell embryos, intensive labeling was seen in interchromatin granules, which, in their turn, were often seen in close proximity to the nucleolus precursor bodies, or in the four-cell stage were interconnected to them. No labeling was ever seen, using antibodies specific for the snRNP Sm antigen, in nucleolar precursor bodies during embryonic nucleologenesis nor in the resulting nucleoli. There was some incidental labeling of the large central vacuole appearing at the beginning of the nucleolus precursor body transformation, testifying the nucleoplasmic origin of this entity.