The high chromosomal instability in New Zealand black mice was greatly reduced (to normal values) in vivo by repeated injections of bovine superoxide dismutase (Cu-SOD) over a period of 3 wk. The decrease in aberrations occurred in all the mice treated, and the SOD appeared to be effective for all types of aberrations, including chromatid gaps, telomeric constrictions, chromatid breaks, telomeric extrusions, and fragments.