Abstract: B iomon ito ring of the geno tox ic po ten t ial of occupat ional and environm en tal facto rs and their b io logical effect s in m an is essen t ial fo r en su ring p rim ary p reven t ion of hum an cancers and dam age of genet icm aterial. The group s of popu lat ion mon ito red in the Czech Repub lic fo r the occupat ional expo su re to clastogen ic geno tox in s comp rise b road spect rum of expo su re, the mo st of them invo lve comp lex m ix tu res of gases, vapo rs, and part icu lar m at2 ters. The increase of the ch romo som al aberrat ion f requency is con sidered as the signal of early adverse b io logi2 cal effect. Recen t resu lt s f rom Scandinavia, Italy and the Czech Repub lic show that an increased level of ch ro2 mo som e b reak s appears to be a relevan t b iom arker of fu tu re cancer risk. In the Czech Repub lic the cytogenet icanalysis has been in u se since 1974 fo rmon ito ring group s occupat ionally expo sed to geno tox ic carcinogen s. Du2 ring th is period m any thou sands of occupat ionally expo sed wo rkers have been exam ined f rom differen t p lan t and types of indu st ry. O ne of these, expo su re to PAH s, num ber of aberran t cells (AB. C. ) w as found betw een 2. 34% and 5. 07%. In popu lat ion group s occupat ionally expo sed to differen t vo lat ile chem ical compounds and their m etabo lites, w ith o r su spected carcinogen ic effect (ep ich lo rohydrin, vinylch lo ride, styrene, fo rm alde2 hyde, bu tadiene) the f requency of AB. C. w as ob served betw een 1. 05% and 4. 71% (con t ro l level 1. 81 % AB. C. ). Tbe ob tained data rep resen t a basis fo r quan t if icat ion of expo su re and fo r the app licat ion of p reven t ive m easu res (e. g. the change ofMAC).