Fly ash was collected from five large-scale or pilot tests in which burning of bleached kraft pulp mill sludge was studied. The content of dioxin-like compounds in this fly ash was estimated both chemically and biologically. Fly ash was analyzed chemically for 17 PCDD and PCDF congeners by high-resolution GC-MS, and the data were transformed to Nordic TCDD equivalents. The biological analyses were based on the induction of several enzymes (aryl hydrocarbon hydroxylase [AHH], 7-ethoxyresorufin 0-deethylase [EROD], aldehyde dehydrogenase-3 [ALDG3]) by the fly ash extracts in a mouse hepatoma cell line, Hepa-1. The inducing potencies were expressed as biological TCDD equivalents. There was a good correlation between the Nordic and the biological TCDD equivalents. Differences in the amounts of dioxin-like compounds among the combustions were attributed mainly to the boiler types and not to fuel characteristics or combustion parameters.