The chromosomal composition of micronuclei (MN) induced by the model mutagens mitomycin (MMC) and colchicine (COL) as well as by acrylamide (AA) and the traditional Chinese medicine Tripterygium hypoglaucum (level) hutch (THH) in NIH 3T3 cells was analyzed by multicolor fluorescence in situ hybridization (FISH) using DNA probes for the centromere repeated minor satellite DNA and the telomeric hexamer repeat (TTAGGG). The majority of MN (78.6%) from treatment with MMC (0.1 microg/ml) did not show centromeric signals, reflecting the clastogenic action of MMC. Following treatment with COL (0.1 microg/ml), 74.5% of the MN showed centromeric signals and several telomeric signals, indicating that MN induced by this well-known aneugen were mainly composed of whole chromosomes. After treatment with AA (100, 200 and 400 microg/ml) both MN containing whole chromosomes and MN containing acentric fragments were found to increase in a dose-dependent manner, demonstrating that AA is not only a clastogen but also an aneugen. THH induced a high frequency of MN harboring whole chromosomes at all concentrations tested (5, 10 and 20 microl/ml) and produced a dose-dependent increase in fragment-containing MN, indicating that THH has both aneugenic and clastogenic potential.