Human pre-embryos, at 2-8 cells, with different morphology have been examined for chromosome constitution. Concerning the informative pre-embryos (23/65), 11 showed a normal karyotype and in 12 an abnormal karyotype was observed; there was not a complete correspondence between pronucleation and chromosome set. Also in the interphasic pre-embryos the presence of chromatin masses at different sizes was revealed. The observation of polyploid and/or polynucleated blastomeres can suggest a failure of synchronization between cytoplasmatic and nuclear division. Mosaicism, polyploidy and polynucleation are phenomena that could be caused by in vitro culture conditions or dependent upon the genetic anomaly of the zygote.