With the introduction in 1970 of DNA staining techniques productive of specific banding patterns in each chromosome, detailed chromosomal morphology became apparent, and precise karyotyping was a reality. To study the banding patterns of malignant tumors, six moderately well-differentiated transitional cell carcinomas of the bladder subjected to the direct squash procedure were stained by the acid-saline-Giemsa technique. Chromosomes A-2, B-4, and D-13 showed the most consistent patterns. Two marker chromosomes characterized the six tumors: marker l, a long submetacentric, was present in 66% of the karyotypes and marker II, resembling a B-4, was present in 28%. These markers, coupled with the subtetraploid mode of the six tumors, were unique when compared with well-differentiated and poorly differentiated bladder carcinomas. Potentially, DNA banding provides a more accurate method of classification, including assay of invasiveness, of malignant neoplasms of the bladder.