A standard protocol for a micronucleus assay in root tips of Zea mays was developed and calibrated using 137Cs gamma radiation, ethylmethanesulfonate (EMS), and N-ethyl-N-nitrosourea (ENU). The root tips were sampled 1, 3, and 5 days after the treatment period. Primary root tips (sampled 1 day after treatment) were the most responsive with the highest frequencies of micronuclei. The frequencies then decreased to control values after 3 days. By treating the same kernels for this assay and the yellow-green-2 (yg2) forward mutation assay, it was possible to analyze and compare the induction kinetics of micronuclei formation in root-tip cells and forward mutation at the yg2 locus in leaf primordial cells. Forward mutation at the yg2 locus by gamma rays, EMS, or ENU exhibited one hit kinetics. The induction of micronuclei demonstrated nonlinear kinetics with a significant increase over controls above 200 rads gamma rays, 10 mM EMS, or 250 microM ENU. ENU induced a higher frequency of micronuclei at all treatment concentrations than did gamma radiation or EMS. The data suggest that a higher relative proportion of yg2 mutant sectors induced by ENU may be due to chromosome aberrations than those induced by gamma radiation or EMS.