The size of micronuclei in human lymphocytes varies according to inducing agent used.

Micronuclei were induced in vitro in human lymphocytes by mitomycin C, X-rays, vincristine, and colcemid and analyzed in cells with preserved cytoplasm. The micronucleus/cell nucleus ratio was measured. It was found that micronuclei induced by mitomycin C and X-rays were significantly smaller than those formed by vincristine and colcemid. Thus, in spite of the wide size span of human chromosomes, it could be shown that it is possible to differentiate between micronuclei formed by spindle-damaging agents (vincristine and colcemid) and those induced by agents directly damaging the chromosomes (mitomycin C and X-rays). Mitomycin C-induced micronuclei were smaller than those induced by X-rays, probably because the former agent preferentially produces chromatid fragments and the latter chromosome fragments.