Nonradioactive in situ hybridization provides a rapid method for detecting specific nucleic acid sequences. In this study of a patient with acute leukemia, we applied in situ hybridization for identification of a marker chromosome and determination of the number of copies of this marker in interphase nuclei using a biotinylated Yq-specific DNA probe (pY3.4). We show that nonradioactive interphase in situ hybridization can be a useful method for karyotypic analysis in addition to routine cytogenetic techniques in neoplastic disorders.