  We investigate effects of galaxy-galaxy interaction on the Tully-Fisher
relation. The HI linewidth in interacting galaxies is significantly broader
than the CO linewidth, and the HI to CO linewidth ratio is proportional to the
strength of interaction. This provides different distances to galaxies measured
by the Tully-Fisher relation with the CO and HI linewidths. Distances derived
from the HI linewidths are 62% larger than those derived from the CO linewidths
for strongly interacting galaxies, and 25% larger for weakly interacting
galaxies. We argue that the CO-line Tully-Fisher relation will be more reliable
to measure the distances of interacting galaxies as well as galaxies in rich
clusters.
