Chromosomal studies were performed on primary cultures from human prostatic adenomas and carcinomas. All cells from both tumor types showed diploid chromosomal counts with a normal human karyotype. Chromosomal analysis was also performed on cells of the permanent epithelial line EB 33 derived from a human prostatic carcinoma in this laboratory. In contrast to the primary cultures, hypotriploid chromosome numbers were found during continuous culture for more than 1 yr. The range of chromosome numbers increased with time in culture and was reduced to the original modal number of 64 by only 1 animal passage (nude mice). Feulgen fluorometry on different passages of the permanent cell line EB 33 showed tetraploidy. This discrepancy to the hypotriploid chromosome number can be explained by different Feulgen hydrolytic properties of the different cell types.