In a continuing study of the effects of inhibition of gluconeogenesis at the phosphoenolpyruvate carboxykinase (PEP CK reaction on in vivo growth of tumors, it has been shown that hydrazine sulfate, a known inhibitor of PEP CK, not only inhibits the Walker 256 intramuscular carcioma, the Murphy-Sturm lymphosarcoma and the B-16 melanoma, but also has an effect on the solid L-1210 leukemia which can be unmasked with the use of an additional inhibiting substance (combination chemotherapy). In addition to hydrazine sulfate various organic hydrazides tested against the Walker 256 intramuscular carcinoma produced varying degrees of tumor inhibition, the most potent being pyridine-2,3-dihydrazide (quinolinic acid dihydrazide) which contains both quinolinic acid and hydrazine, both of which can separately inhibit PEP CK; the latter is thought to account for this compound’s augmented antitumor action. Evidence is presented that tumor inhibition and toxicity (weight loss) may share a common mechanism of action, but if so that these two functions can be separated or toxicity minimize