Gaining knowledge about the physiological timetable of gene expression during preimplantation embryo development is crucial, and a better understanding of cytokine and growth factor expression in early embryonic development could lead to improved in vitro culture conditions and enhance in vitro fertilization implantation rates. Our aim was to detect the patterns and levels of two messenger ribonucleic acids [mRNAs; beta-actin and interleukin-1 receptor type I (IL-1R tI)] in single human blastomeres by RT-nested PCR and to compare possible variations in the gene expression both between different embryos and in multiple blastomeres within the same embryo. Single blastomeres from nine human tripronucleic preimplantation embryos were examined by one round of RT and two rounds of nested competitive PCR. Beta-actin mRNA was detected in each blastomere, and IL-1R tI mRNA was found in 72% of the blastomeres examined. Beta-actin was expressed at a level of 511-12185 molecules of complementary DNA/blastomere, and IL-1R tI was expressed at a level of 2-290 molecules of complementary DNA/blastomere. Our results suggest that the mRNA pattern of an embryo cannot be reliably quantitated from the mRNA pattern of a single blastomere and therefore imply limitations for the use of this method for preimplantation diagnosis.