Human peripheral lymphocytes (HPLs) are noncycling primary cells (G0 cells). They are easily collectable by venipuncture. In the presence of suitable culture media and stimulants, in vitro HPLs enter the cell cycle and divide mitotically. Metaphase-like stages can be arrested using the spindle fiber poison Colcemid and prepared on microscopic slides. Following appropriate staining, chromosomal aberrations can be analyzed in the microscope. These aberrations may either be induced in vivo by environmental or occupational influences or in vitro after experimentally controlled manipulations in order to detect or to test the mutagenic potency of various agents.