Two karyotypically stabilized callus strains of Crepis capillaris (2n = 7 and 2n greater than or equal to 12, respectively) were employed to illustrate the utility of plant tissue culture method for screening and analysis of cytogenetic effects of chemicals following long-term treatment. The cytogenetic analysis of callus cells revealed significant differences in the toxic and mutagenic effects of chemicals under study (2,4-D, MH, NMU and kinetin). A certain dose-response relationship (though not necessarily linear) was observed. The maximum cytogenetic activity of chemicals was associated with certain intermediate concentrations (4.5 X 10(-5) M-9 X 10(-5) M), whereas a further increase in the dose either gave rise to the opposite effects (i.e. decrease in the ana- and telo-phase aberration rates and increase in the modal karyotype frequency) or the aberration rate remained unchanged.