HAPAB A review is presented of chemical agents which break chromosomes, referred to as chemical ''clastogens''. Pesticides which have been found to be clastogenic in human cells, in vivo or in vitro, include: arsenicals, carbon tetrachloride, nitro compounds, ethylene oxide, lindane, lead, Apholate (aziridine), Captan, Hemel (HML), Hempa (HMP), Tretamine (TEM), triethylenephosphoramide (TEPA), triethylenethiophosphoramide (thio-TEPA) and trimethylenephosphoramide (TMPA). Some substituted derivatives of melamine or phosphoramide employed as insect chemosterilants are also extensively used as antileukemic drugs and as carcinostatic agents. DDT, although mutagenic in nonmammals, has not yet been proven to be clastogenic in the concentrations tested. The stable rearrangements resulting from chromosome breakage concern the physician most because they can produce unbalanced genetic constitutions and may be passed on to future generations. The spontaneous abortion rate or the mongol birth rate are suggested as possible minitors for human ''genetic catastrophe.''== 1970