Namalva cells were propagated continuously over a period of up to 18 months. During this period the chromosomal status of the cell populations were investigated cytogenetically. The ability of the cells to produce interferons after induction with Sendai virus was monitored. In contrast to the drastic chromosomal variation observed, interferon production was remarkably stable. Comparison of the various cytogenetic data revealed the presence of marker chromosomes and chromosomal constellations which were excluded from the drift. Some of these are useful for unequivocal identification of Namalva cells during long term cultivation.