Chromosome preparations were made from 15 cleaved human embryos in the 2- to 12-cell stage after in-vitro fertilization. All showed two pronuclei before the first cleavage. Twelve had at least one diploid metaphase, while three had interphase nuclei only. There was no evidence of parthenogenetic haploid cleavage. As expected, the frequency of metaphases increased with duration of colchicine treatment: 25 and 55% of the cells reached metaphase after 6-13 and 16-24 hours' treatment, respectively. Cleaved embryos with 'ideal' blastomere numbers (2, 4 or 8) showed a considerably higher metaphase frequency than others. For a more detailed chromosome analysis the technique will have to be further improved. The asynchronous cleavage of blastomeres makes optimal treatment by mitogens difficult.