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INTRODUCTION: The in vitro chemosensitivity testing aims at predicting the response of an individual tumour to chemotherapy choosing optimal agents for a particular patient. Among many chemosensitivity tests developed over the years [1-6], special emphasis was made on clonogenic assays that showed good use and correlation between laboratory and clinical data [7-9]. One of the assays used to predict the response to various anti-cancer modalities is the micronucleus assay using the cytokinesis-block [12-14]. This block is achieved by administration of Cytochalasin-B in order to prevent cytoplasmic, but not the nuclear, division. This leads to micronucleus formation which are counted in binuclear cells. Since there are only a few reports of the use of this assay in predicting chemosensitivity [13, 16], we explored the possibility of using this assay to predict chemosensitivity to various anti-cancer agents. MATERIAL AND METHODS: Exponentially growing SCC VII cells were treated with various concentrations of 11 anti-cancer agents: Mitomycin C, Doxorubicin (ADR), Epirubicin (EPI), Cisplatin, Carboplatin (CBDCA), Etoposide (VP-16), Vincristine, 5-fluorouracil, Methotrexate, Nimustine, and Dacarbazine for 1 hour. After that, Cytochalasin-B was added and dishes were incubated. After various time intervals, cells were fixed in situ and dried. Electron microscope was used to count the number of micronuclei (MN) in binucleate cells as well as multinucleate cells (MNC) in the total cell population. Cell survival was also evaluated by using the colony formation assay [18]. RESULTS: Maximal % of binucleate cells (BNC) was usually reached at 24-30 hours of culture, except for cells treated with ADR and EPI, in which it was reached at 30-72 hours (Figures 1 and 2). All drugs induced formation of micronuclei and dose-response curves for micronucleus frequency were obtained using the data at peak % BNC times. For all drugs, micronucleus frequency increased with concentration (Figure 3), but at the highest concentration used (considered to be overly toxic-Figure 4), the micronucleus frequency was rather lower. This decrease in micronucleus frequency was largely attributed to the decrease in % BNC. When the data at the highest concentrations of all drugs were excluded, a correlation was found between micronucleus frequency and surviving fraction (r = 0.85; p < 0,001) (Figure 5). DISCUSSION: Since micronucleus formation is a sign of chromosome damage that leads to cell death, we used this assay to evaluate chemosensitivity in 11 widely used anticancer agents. Although they can be classified according to mechanism of action as different class agents, they have in common the formation of micronuclei as a sign of cytotoxicity. Cell cycle arrest observed in some agents might be evaluated by assessing the delay in increase of BNC and MNC. The difference observed regarding cell cycle arrest suggested different mechanisms of its action. MN frequency was almost dose-dependent at lower concentrations, but at the highest concentrations, it obviously decreased, showing, therefore, some discrepancies with the data obtained when radiosensitivity was tested that way [14], probably due to extreme toxicity of agents. The optimal concentrations seem to be those providing a 20-80% surviving fraction. Another slight difference, when compared with similar radiosensitivity studies is a decrease with longer duration of culture observed in chemosensitivity testings. The reason for this difference is still unknown, but it emphasized the necessity for choosing the optimal duration of culture, probably that necessary for reaching maximal % BNC. This assay seems useful in predicting chemosensitivity of at least some tumour cells to various (appropriate) concentrations of various anti-cancer agents. However, new studies are warranted to further use of this assay, before testing it in clinical practice.