Quadriradial (QR) configurations from four different human lymphocyte metaphase samples have been analyzed: a patient with Fanconi's anemia; normal female cells X-irradiated with 150 or 200 R in S or G2; spontaneous QRs occurring in 13,584 metaphases; and cells from two sibs with Bloom's syndrome. That mitotic chiasmata are caused by crossing-over rather than by random breakage and reunion was concluded from the following observations: (1) In the spontaneous sample, mitotic chiasmata are about as frequent as all other QRs together. (2) The frequencies of mitotic chiasmata and of other QRs are not correlated in the different samples. (3) The break points in other QRs are situated at random relative to chromosome length, whereas the distribution of chiasmata is highly nonrandom. (4) Although the centromeres of chromatid translocations occur in alternate and adjacent positions with approximately equal frequencies, there are very few adjacent counterparts to mitotic chiasmata. These can best be interpreted as a result of an abnormal U-type rejoining of chromatids in a chiasma. (5) Chiasmata found in heteromorphic chromosome pairs show that crossing-over has, indeed, taken place.