Methylation pattern peculiarities revealed by immunocytochemical analysis of metaphase chromosomes from preimplanted human embryos with monoclonal antibodies against 5-methylcytosine are described. Chromosomes of 2-8-cell triploid human embryos are undermethylated, if compared to those from PHA-stimulated fetal cord blood lymphocytes. Hemimethylation (asymmetric labeling of sister chromatids) is typical for the most of embryonic chromosomes at 2-cell--blastocyst stages due most probably to a passive loss of methylation during initial cleavages. Diffuse labeling and sister chromatid exchanges are two other cytogenetic peculiarities revealed by immunofluorescent staining of early human embryos. Hypomethylation of pericentromeric heterochromatin of chromosomes 1, 9, 16 and different methylation status of some homologous chromosomes may distinguish them from metaphase chromosomes of lymphocytes. M-banding pattern typical for chromosomes from adult and cord blood lymphocytes initially appears in embryonic metaphase chromosomes as early as at a 8-cell stage to be established for most part of chromosomes of the karyotype at the morula-blastocyst stage only.