For individuals with multiple exposures to bone marrow mutagens the glycophorin A (GPA) assay appears to detect the cumulative genotoxic effects of these nonspecific exposures. To determine whether workers exposed to organophosphates and/or carbamate pesticides have more GPA variants compared with controls, this cross-sectional cohort study examined 67 Latino farm workers, most of whom were exposed to organophosphate or carbamate pesticides, and 68 age-, gender-, ethnicity-, and education-matched reference subjects. Of the 27 participants who completed the questionnaire and had the M/N alleleotype required for the GPA assay, the N/N variant frequency was 9.1 x 10(-6) in the ten subjects with more than 1,500 cumulative hours of exposure vs 3.8 x 10(-6) in the six referent subjects with no known pesticide exposure (p = 0.097). A consistent trend with increasing cumulative exposure was found for N/N and O/N allele-loss frequencies when exposures were stratified into three groups. Despite the small sample size due to the alleleotype restriction, these findings support the hypothesis that some pesticides are mutagenic to hematopoietic stem cells.