The clastogenicity of lobeline and possible interactions between lobeline and ethyl alcohol were investigated in a mutagen-sensitivity assay on cultures of human lymphoblastoid cell lines. Lobeline (10(-6) M) and ethyl alcohol (1% and 2%) were added to the cultures alone or in different combinations prior to or simultaneously with bleomycin treatment. The number of chromatid breaks per cell served as the indicator of clastogenicity. Lobeline alone was not clastogenic, but there was a marked increase in genetic damage resulting from a coclastogenic interaction between lobeline and ethyl alcohol. These data emphasize the significance of interactions between compounds that may increase genotoxicity in cells exposed to mutagenic environments.