Fine particles (PM2.5) emitted from municipal solid waste incineration (MSWI) contain high amounts of toxic compounds and pose a serious threat to environment and human health. In this study, entire particles as well as extracted water-soluble and-insoluble fractions of PM2.5 collected from MSWI and biomass incineration (BMI) were subjected to physiochemical characterization and cytotoxic tests in A549 and BEAS-2B cells. MSWI PM2.5 had higher contents of heavy metals (including Pb, Zn, and Cu) and dioxins (PCDD/Fs) than did BMI PM2.5. The metals were enriched in the water-insoluble fraction, as measured by inductively coupled plasma-atomic emission spectrometry. BMI PM2.5 had a higher content of endotoxin, which was also enriched in the water-insoluble fraction. MSWI PM2.5 caused more serious cell injuries, as indicated by the lower viability, higher ROS generation, and DNA damage, whereas BMI PM2.5 presented higher pro-inflammatory potential, as indicated by increased mRNA levels of interleulcin 6. Normal human BEAS-2B cells were more sensitive than A549 cells in all these tests. Toxic effects caused by MSWI and BMI PM2.5 were mostly attributable to their water-insoluble fractions. Our results indicate different chemical and biological compositions in MSWI and BMI PM2.5 probably dominate in different toxic endpoints in vitro.