Micronuclei (MN) in erythrocytes of mouse bone marrow cells were induced in vivo by the spindle poisons colchicine (COL) and vinblastine (VBL), by hydroquinone (HQ) and by the alkylating agent mitomycin C (MMC). Two different methods were applied to detect whole chromosomes with centromeric proteins or chromatin in MN to discriminate between spindle damaging or clastogenic activity of these chemicals. One method determined the fraction of MN with centromeric chromatin by immunofluorescent staining using antikinetochore antibodies (CREST staining). The other method applied non-radioactive in situ hybridization with a novel DNA probe. The fractions of MN that showed positive signals by either technique thus indicating with a high probability the presence of whole chromosomes instead of acentric fragments, were in good agreement for COL, VBL and HQ. After application of MMC, however, 4.5% of the MN were CREST-positive, while 29% gave a positive hybridization signal. The results suggest, that kinetochores may have lost certain centromeric antigens due to treatment with MMC so that MN containing whole chromosomes appear CREST-negative. The presented in situ hybridization scheme using satellite DNA is a more direct detection and is advantageous to the CREST staining technique in that it is unaffected by damage of kinetochore or centromeric function.