The cytological behavior of the spindle apparatus was studied in cells prone to nondisjunction (ND), i.e., PHA-stimulated lymphocytes derived from children suffering from different types of neoplasia. These cells, which exhibited a high frequency of nonspecific aneuploidy, revealed an increased resistance of the spindle fibers to colchicine, podophyllotoxin, and cold, which was several times that of lymphocytes derived from healthy children. The results are in accord with previous findings showing a high resistance of spindle microtubules to the antimicrotubular agents colchicine, podophyllotoxin, vinblastine, and cold in PHA-stimulated lymphocytes derived from individuals prone to meiotic ND. It is therefore assumed that high resistance of the spindle apparatus to antimicrotubule agents characterizes cells at high risk for aneuploidy, and possibly, the overstabilized spindle fibers are responsible for failure of chromosomal disjunction.