The effect of a zinc-deficient diet on serum and liver zinc levels was studied in BALB/c mice in the presence and absence of the IgM-secreting plasmacytoma, TEPC-183. While serum zinc levels were significantly decreased in the zinc-deficient (ZD) mice by Week 2, there was no change in the level of zinc in the liver of this group compared to ad libitum (AL) or pair-fed (PF) controls even after 4 weeks on the respective diets. The presence of the tumor itself resulted in a significant decrease in serum zinc levels in mice maintained on a normal-chow diet. This decrease was not seen in the serum from tumor-bearing AL or PF control mice which were fed a synthetic diet containing 50 ppm zinc. There was, however, an increase in the liver weights of mice in these groups in the presence of TEPC-183 which was not seen in the ZD tumor-bearing mice. Although total zinc levels increased in the livers of AL and PF mice reflecting this increase in liver weight, there was no difference among the groups when the amount of zinc was determined per gram of liver tissue. There was also no difference in the amount of zinc per gram of tumor tissue in the tumors obtained from AL, PF, or ZD mice.