Fluorescent properties of two naphthalimides and a phenalenone derivative in organic solvents and when they bind to human peripheral blood lymphocytes were investigated. Different spectral characteristics were observed using lymphocytes of healthy donors and patients with nonmalignant (chronic myeloid leukemia) and malignant (B-cell lymphoid leukemia) diseases. It was found that spectral properties of the used fluorophores in cell suspension qualitatively characterize its structural and functional alterations during pathological phenomena. The intensity of fluorescence increased in samples from patients with B-cell lymphoid leukemia, and the fluorescence maximum shifted to the long-wavelength region by 20 nm compared with normal lymphocytes. It is concluded that 3-isopropyloxy-6-morpholino-2-phenylphenalen-1-one as most promising probe may be applied to the study of malignant diseases.