The effect of pyridoxine depletion on the expression of serum protein species in control and Morris hepatoma No. 7777-bearing rats was studied with polyacrylamide gel electrophoresis (PAGE). A group of 20 Buffalo female rats was fed ad libitum a complete diet lacking pyridoxine, whereas a similar group was fed the same diet with pyridoxine. After a 22-day feeding period, 12 animals from each group were inoculated with hepatoma No. 7777 cells in the thigh muscles of both hind legs and allowed to grow for 25 days. Sera were obtained from all animals by heart puncture under light ether anesthesia, and protein species were resolved by PAGE. Eight and four protein bands in the haptoglobin and postalbumin regions, respectively, were resolved from control rat serum, whereas sera from depleted animals had eight and three bands, respectively. Protein expression was facilitated by the presence of hepatomas. Ten and six protein bands were seen in the haptoglobin and postalbumin regions, respectively, upon PAGE of sera from control tumor-bearing rats. A positive synergistic effect between pyridoxine lack and tumor presence was also found. Twelve and five bands were resolved in the haptoglobin and postalbumin regions, respectively, when sera from pyridoxine-depleted tumor-bearing rats were electrophoresed. These results showed that vitamin B6, in addition to its well-known coenzymatic function, exercised a type of control over protein expression, which may be positive or negative depending on the nutritional and health status of the animal.