The herbicides alachlor and maleic hydrazide were evaluated for genotoxicity in peripheral blood human lymphocyte cultures. Sister-chromatid exchanges (SCE), chromosome aberrations (CA) and micronuclei (MN) were scored as genetic endpoints. To detect possible metabolic modifications in the genotoxicity of both herbicides, the cultures for SCE and MN demonstration were also treated with S9 fraction. From our results we conclude that, in the absence of metabolic activation, the two herbicides induce significant increases in the frequency of SCE, although the concentrations needed to be effective are very different. Thus, alachlor gave positive results at concentrations ranging from 1 μg/ml, and maleic hydrazide at concentrations ranging from 100 μg/ml. In addition, alachlor appears to be clastogenic in both the CA and MN assays, but only at the highest concentration tested (20 μg/ml). The co-treatment with the S9 fraction produced a slight decrease in the induction of SCE with both herbicides; nevertheless, it does not seem to affect the response in the MN assay.