To question the possible implication of an alteration of the DNA methylation of imprinted genes in normal development failure observed following fertilization in ART centers, it has been necessary to develop a reproducible and highly efficient method to perform analysis at the one cell level. We have thus developed a very efficient protocol for methylation studies on individual oocytes or cleavage-stage embryos. All the different steps were optimized, from DNA extraction, to limit DNA degradation and give a high success rate of bisulfite converted DNA, to amplification of the bisulfite modified DNA.