In immunocytochemical studies, the phenotypic evaluation of tumor cells is often complicated by accompanying normal cells, representing the original tissue or infiltrating leukocytes. This holds particularly true for tissues with a great morphological and immunophenotypical variability, such as bone marrow. A method that identifies mitotic tumor cells by duomosomal aberrations and permits the subsequent immunophenotypical analysis was a fmt progress, demonstrated by Teerenhovi et al. However, the results are usually hampered by the low number of analyzable mitoses. We dem-

Recently, we have presented a new technique for immunophenotyping cells that have numerical chromosome aberrations. We referred to this method as “Fluorescence-Immunophenotyping and Interphase Cytogenetics as a Tool for Investigation of Neoplasms” (FICTION). We present here an advanced FICTION method with three-color staining and improved sensitivity.

We describe a method that enables detection and immunophenotypical characterization of distinct subpopulations within a cytogenetically defiied tumor clone. CoariSting normal cells do not hinder microscopic evaluation because they can be distinguished from cytogenetically aberrant tumor cells. This is also true when normal and neoplastic cells cannot be dearly distinguished by cytology or immunohistochemistry, i.e., if both constituents have similar immunophenotypes and morphology. The method is based on fluorescence double staining for two Werent antigens com-

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