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The cytogenetic effect of various genotoxic compounds was analysed in cytokinesis-blocked human peripheral lymphocytes and V 79 cells by using immunofluorescence and micronucleus methods. Cytochalasin-B (CYB) induced a maximum number of binucleated cells at 3-4 {dollar}\\mu{dollar}g/ml without any genotoxic effect. Comparative studies between binucleated (CB) and mononucleated micronucleus assays show that the micronucleus (MN) frequency in CB method was higher than those in the mononucleated cells (P {dollar}<{dollar} 0.05). The MN frequencies in cultures treated with ethylene dibromide (EDB) and vincristine sulfate (VS) were scored. VS caused significantly higher MN frequencies than controls (P {dollar}<{dollar} 0.01) at 24 h and decreased later dramatically with time. VS induced a large number of micronucleated cells with multiple MN. Continuous presence of VS in cultures caused a severe decrease of nuclear division index (NDI). EDB induced MN in binucleated cells in both 4 h and continuously treated cells. EDB induced very few micronucleated cells with multiple MN and it did not affect nuclear division. Immunofluorescent staining of kinetochores in MN was found to be a useful technique for distinguishing clastogenic and aneuploidogenic agents. Within the base line MN (2%) 50 to 58% of cells contained kinetochore-positive (Kc+) MN, indicating the origin of MN from both acentric and centric chromatid(s)/chromosome(s). Mitomycin C (MMC) induced a significantly higher number of MN than controls (P {dollar}<{dollar} 0.01) and the majority of micronucleated cells contained kinetochore-negative (Kc{dollar}-{dollar}) MN, indicating that MMC is a clastogenic agent. VS also produced statistically significant number of MN (P {dollar}<{dollar} 0.01). However, the majority of micronucleated cells contained Kc+ MN indicating that VS is an aneuploidogen. Cigarette smoke condensate (CSC) induced a statistically significant number of MN (P {dollar}<{dollar} 0.01). CSC induced both Kc+ (P {dollar}<{dollar} 0.01) and Kc{dollar}-{dollar} MN (P {dollar}<{dollar} 0.05) indicating that CSC is both a clastogenic and an aneuploidogenic agent. These results suggest that immunofluorescent staining of kinetochores in binucleated cells is capable of distinguishing between clastogenic and aneuploidogenic agents.