Three placental tissue cultures of spontaneous human abortions showed an unusually high frequency of metaphases with diplochromosomes. In 62 such cells, nine configurations were interpreted as mitotic chiasmata between the two sister chromosomes of a diplochromosome. One U-type exchange between two sister chromosomes was also found. This differs significantly from the 1 : 1 ratio of adjacent and alternate exchanges in translocations, thus supporting the idea that mitotic chiasmata are in principle different from chromatid translocations. The hypothesis is put forward that the frequency of homologous exchanges is determined by the intimacy of pairing which ranges from meiotic pairing through sister chromatid association, through sister chromosome association in diplochromosomes to accidental pairing of homologous regions in diploid cells.