When V79 metabolic cooperation (MC) assay was performed to detect tumor-promoting activities, inhibitory activities on the gap-junctional intercellular communication were first detected in both extracts prepared from polyetherurethane (PEU) and silicone. The former inhibited more strongly than the latter. Furthermore, the lowest effective concentrations in MC assay correlated well with the values of the total active incidences obtained by histologic evaluation of the 2-year implantation test in rats. Poly(tetramethylene oxide) (PTMO) showed the highest inhibitory activities among the constituents of PU. Thus, the shorter the chain length of PTMO, the stronger the inhibitory activities of PTMO in MC assay. PTMO moiety may play an important role in the tumor-promotion stage during tumorigenesis induced by PU.