In this study we demonstrate the feasibility of chromosomal in situ suppression (CISS) hybridization to detect the translocation t(15; 17) in metaphase spreads of patients with acute promyelocytic leukemia. Using DNA libraries from sorted human chromosomes 15 and 17 the translocation t(15; 17) can be unequivocally identified even if the spread and the morphology of the chromosomes are poor. The sensitivity of CISS hybridization is compared with the sensitivity of conventional G-banded karyotypes.