Two hundred and thirty three-isolated blastomeres originating from 82 abnormal human 2 to 11-cell embryos were fixed separately for cytologic and cytogenetic analyses. These embryos were rejected from our in vitro fertilization program because of either the presence of 1 or 3 pronuclei when observed 17 hours after insemination, or a wide fragmentation of normally fertilized eggs, or delayed fertilization. Twelve per cent of the blastomeres had no nuclei, whereas 7% were binucleated. When comparing the different types of embryos, parthenogenetic ones were those displaying the closest to normal development, since only 3% were cytologically abnormal. Seventy-six percent of the embryos had at least one analysable mitosis. Only 33% had 2 analysable mitoses, and 11% at least 3. Embryos with more than 1 analysable mitosis were the most often mosaicisms. When compared with classical techniques involving the fixation of whole embryos, cytogenetic analysis of isolated blastomeres permits ensuring that the number of chromosomes per cell is not overestimated as a consequence of mixed mitoses and demonstrates the very high incidence of mosaicism in abnormal human embryos.