The cells of two ascitic effusions from human cancers of the ovary were analyzed by the fluorescence and Giemsa banding techniques. The normal chromosomes still present in the cancer cells were identified, and several chromosomes which appeared normal with the conventional techniques were found to be rearranged ones. In the rearranged chromosomes it was possible to identify chromosome material which appeared preferentially lost. The sequence of events leading to the formation of many new chromosomes were deletions, inversions, and translocations. Both ascites showed, in all cells, a relative constancy in the ratio of normal to new chromosomes, and this suggests that not all the possible combinations of normal and abnormal chromosomes can occur. In one of the ascites, six different rearrangements have been found in several cells, all involving the long arm of chromosome No. 1.