In investigations using experimental animals, the unexpected affection of c
ertain diseases often cause great impairment to them especially when using
aged animals. In F344 rats, large granular lymphocyte leukemia is the most
frequent fatal disease which increases along their aging. The timely detect
ion of rats at risk for leukemia is very important in order to exclude such
animals and thus obtain precise results in many fields of investigation. I
n the process of assessing the main cause of death in F344/N rats of the ag
ing farm of our institute, NILS, we found cases with no obvious contributor
y disease to death that showed anisocytosis in a peripheral blood smear. In
such cases, histological examination of spleen revealed consistent feature
s of leukemia and findings of the liver and kidney were considered due to t
his hematologic disorder. Anisocytosis was frequently seen in the advances
stage of leukemia. Thus we concluded that anisocytosis is a prior condition
of leukemia and its detection in a peripheral blood smear is predictive of
the disease when using aged animals. (C) 2000 Elsevier Science Ireland Ltd
. All rights reserved.