Chromosome preparations using hypotonic pretreatment and air drying have been examined from 5 different human euploid fibroblast cultures. Between 0.1 and 4.1% of mitoses are found to be tetraploid with the chromosomes arranged as diplochromosomes. 0.5–5.8% of the mitoses are tetraploid with randomly positioned chromosomes. Differences in the frequency of polyploid mitoses between two cultures set up from one biopsy may be greater than between cultures from different individuals. The structure of the chromosomes forming diplochromosomes is identical to that of the chromosomes in diploid mitosis. In prophase the two chromosomes of a diplochromosome lie close together but are not linked at the centromere. In late prophase and metaphase the two chromosomes of a diplochromosome usually remain together but sometimes lie apart. In late anaphase and telophase no pairing of daughter chromosomes can be found. In prophases of cells from females, one of the chromosome pairs, presumably an X-diplochromosome, may show heteropyknosis. It is assumed that diplochromosomes originate mainly through endoreduplication. The mitosis following a mitosis with diplochromosomes will presumably show the chromosomes in random position. Autoradiographic studies using 3H-thymidine show that diplochromosomes originate from two successive chromosome duplications. The cycle leading to diplochromosomes has approximately the same duration as two cycles of a diploid cell. The late labelling pattern, especially the late labelling X-chromosome in cells of females, is found in diplochromosomes in the same order as in chromosomes of diploid cells. The grain pattern of diplochromosomes labelled during first chromosome duplication indicates a semi-conservative mode of segregation.