The frequency of anaphase abnormalities (usually presented as bridges, bridges plus fragments, or percentage of normal anaphases) is widely used as a criterion of radiation damage in many materials. Here, the frequency of these three abnormal anaphase categories has been studied in a material frequently used for this purpose, regenerating mouse liver, following fast neutron irradiation (50 to 800 rads). A previous study with the plants Tradescantia and barley had shown that detected anaphase bridges increased with dose more or less linearly at first, to a maximum, and then actually declined at higher doses. Two different bridge-loss processes between metaphase and anaphase were shown to account quantitatively for this dose response. The results and conclusions were felt to be applicable to anaphase data in general, from any material. The same response was found here in the mouse liver. The frequency of anaphase bridges per cell increased with dose to a maximum, then declined at larger doses. The percentage of normal (aberration-free) anaphases reflected this increasing-with-dose loss of bridges by showing an actual increase in normalcy at larger doses, after an initial decline to a minimum of normalcy. In addition, for the mouse liver cells, it is shown that only 40% or less of the fragments actually present at anaphase are detected. In the practical sense, the widely used criterion of abnormal anaphases is an inefficient but reasonable measurement of damage in the range of low dose and low level of effect; that is, the amount of effect is about proportional to the dose. For mouse livers, it is estimated herein that the observation of abnormal anaphases detects only about 40 to 50% or less (about 40% of fragments, 50% of bridges) of the aberrations actually present, up to a dose of about 200 rads of fast neutrons. At higher doses, as the exponential-with-dose disappearance of bridges becomes appreciable, abnormal anaphases become grossly incorrect, actually decreasing with dose.

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