Little is known about the gene expression in human oocytes and early embryo development because of the rare availability of the materials. The recent advancement of biotechnology has allowed one to analyze the gene expression in single human oocytes and preimplantation embryos. Gene expression of human lactate dehydrogenase isozymes (LDH-A, LDH-B, and LDH-C) and small ubiquitin-like modifier isoforms (SUMO-1, SUMO-2, and SUMO-3) in four oocytes, two 4-cell and three 8-cell embryos was studied using the reverse transcription-polymerase chain reaction. The mRNAs for SUMO-1, SUMO-2, SUMO-3, and LDH-B (heart) were detected in all of oocytes, 4- and 8-cell embryos. The mRNA for LDH-A (muscle) was detected in two of four oocytes and one of three 8-cell embryos. However, the mRNA for testis-specific LDH-C was not detected at all as expected. A cDNA microarray containing 9600 cDNA spots was used to investigate differential expression of human genes in oocyte, 4-cell and 8-cell embryos. The expression of 184, 29, and 65 genes was found to have a value more than twofold above the median value of all genes expressed in oocyte, 4- and 8-cell embryos, respectively, indicating that the expression of some zygotic genes had already occurred at 4-cell embryo.