We describe a method that enables detection and immunophenotypical characterization of distinct subpopulations within a cytogenetically defined tumor clone. Coexisting 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 clearly 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 different antigens combined with interphase cytogenetic analysis. It is referred to as "Fluorescence immunophenotyping and Interphase Cytogenetics as a Tool for Investigation of Neoplasms (FICTION)." In a case of follicular lymphoma we demonstrate that FICTION can differentiate bcl-2-positive malignant and non-malignant cells and can verify the presence of bcl-2-positive but cytogenetically inconspicuous T-lymphocytes.