The Japanese Kanechlor technical PCB formulations such as KC-300, KC-400, KC-500, KC-600 and KC-1000 have been examined for possible contamination with by-side PCDD/Fs. 75 PCDDs and 135 PCDF have been determined using isotope dilution, separation and enrichment on silica gel impregnated with activated carbon, and final HRGC/HRMS measurement. MonoCDDs to OCDD were absent in KC-300, KC-600 and KC-1000. Tetra- and PentaCDDs occurred at > 1 ng/g in KC-400 and KC-500. The Kanechlors were contaminated with nearly all 135 PCDFsw. In parallel with an increasing degree of chlorination of a particular Kanechlor formulation examined increased also the content of more chlorinated PCDFs. In term of total dioxin-like toxicity and TEQ loads the KC-500 contained highly toxic PCDD/Fs at 270 ng TEQ/g and followed by KC-400 with 269 ng TEQ/g, KC-600 with 188 ng TEQ/g, KC-1000 with 164 ng TEQ/g and KC-300 with 79 ng TEQ/g. From 99.5 to 100% of PCDD/Fs toxicity found in the Kanechlors was from PCDFs.