Abstract The toxic equivalent (TEQ) concept is widely used to assess toxicity potential of a dioxin-like chemical mixture. The TEQ approach converts concentrations of various dioxin-like compounds into a single concentration that is toxicologically equivalent to the most toxic dioxin compound, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), using toxic equivalency factors (TEFs). It has been shown that in the absence of costly measurements of dioxin-like polychlorinated biphenyls (dl-PCBs) in fish, relatively inexpensive measurements of total PCB can be utilized to estimate dl-PCB–related TEQ (i.e., TEQdl-PCB). The present study assesses the impacts of uncertainties in dl-PCB measurements and estimates, and mammalian TEFs on TEQdl-PCB using the Monte Carlo technique. The analysis suggests that measurement errors for dl-PCBs translate into up to 1.3-fold uncertainty in TEQdl-PCB, while uncertainties in estimates of dl-PCBs generally produce up to a threefold uncertainty in TEQdl-PCB. In contrast, the uncertainty due to TEFs normally ranges 10- to 13-fold and spans over 30- to 40-fold under extreme cases. For 2005 TEFs, PCB-126 is the dominating contributor to uncertainty in TEQdl-PCB. When we considered uncertainties in the TEFs and estimated dl-PCB concentrations simultaneously, there was little increase in uncertainty in TEQdl-PCB that was already produced by the TEFs only. These results indicate that the dl-PCB composition in fish and/or the relationship between total PCB and TEQdl-PCB can be utilized to estimate TEQdl-PCB with reasonable confidence.