With a commercially available balanced 10% amino acid solution as control (I), imbalance solutions were prepared by the removal of valine (II), valine and leucine (III), valine and isoleucine (IV), and valine, leucine, and isoleucine (V). The solutions, in combination with total parenteral nutrition, were given to rats with AH-109A hepatoma for 7 consecutive days to evaluate antitumor activity. Although there were no significant differences in carcass weight after treatment among groups I-IV, a decrease was noted in group V. Tumor weights in groups II-V, relative to that in group I, were 39, 57, 43, and 44%, respectively. Antitumor activity was thus noted in all imbalance groups, although activity in group III was lower than that in group II. These results indicate that the concurrent depletion of leucine and/or isoleucine along with valine is undesirable because of lower antitumor activity and an adverse effect on host development. Leucine is thus suggested to play an important role in augmenting the activity of valine-depleted imbalance.