To evaluate workers' exposure to dioxin-like substances (such as polychlorinated dibenzo-p-dioxins, dibenzofurans, and polychlorinated biphenyls), the authors used a chemical-activated luciferase gene expression (CALUX) assay to determine serum dioxin-like activity in five workers before and after two different cleaning-up activities inside a municipal domestic solid-waste incinerator. The workers' mean serum concentration of dioxin-like substances before the first cleaning operation, shown as a weighted value of toxic equivalents (or TEQs) according to the CALUX test, was 17.2 pg CALUX TEQ/g fat (range = 12-22), which is comparable with concentrations found in similarly aged men in a Flemish environmental health pilot study. After cleaning work, the workers' mean serum concentration was 28.5 pg CALUX TEQ/g fat (range = 18-31). At the second plant stoppage, the workers' mean dioxin-like activity was 15.4 pg CALUX TEQ/g fat (range = 12-21) before and 16.4 pg CALUX TEQ/g fat (range = < 10-32, where 10 pg is the limit of determination) after the cleaning operation. These results indicate that workers may be exposed to dioxin-like substances during their performance of cleaning operations in a municipal domestic solid-waste incinerator.