Micronucleus frequencies were analysed in an X-irradiated human melanoma cell line and in untreated rectal carcinoma cells. As a special aspect of the micronucleus formation, micronuclei-containing kinetochores were analysed by the method of indirect immunofluorescence. The incidence of kinetochore-positive micronuclei was taken as a measure of chromosome loss. In order to show kinetochores, an anti-kinetochore serum of a CREST syndrome patient was used. In irradiated melanoma cells the mean number of micronuclei increased linearly with increasing dose whereas the proportion of kinetochore-positive micronuclei decreased. The analysis of the time-dependent formation of micronuclei revealed an increase of the micronucleus frequency from 24 to 48 h after irradiation (p.r.) and a slight decrease from 48 to 72 h p.r. The highest number of kinetochore-containing micronuclei was scored 72 h p.r. In rectal carcinoma cells a high rate of spontaneously formed micronuclei was observed. At least 30% of these micronuclei contained complete chromosomes, as indicated by the kinetochore-positive staining. We have attempted to summarize the data by means of a numerical expression which takes into account the relation between lost chromosomes, visible as kinetochore-positive micronuclei, and the total sum of micronuclei.