Since the original discovery of a tumor inhibitory factor in guinea pig serum by Kidd (1953) and its identification as L-asparaginase by Broome (1961), this enzyme has been used to regress tumors, especially lymphomas, of the mouse, rat, dog, and man. The work of Sobin and Kidd (1966) showed that protein synthesis was the first biochemical system to be inhibited by asparaginase treatment of a susceptible tumor. Broome and Schwartz (1967) reported that resistant tumors have a greater endogenous supply of asparagine. Furthermore, Patterson and Orr (1967) found that an asparaginase resistant form of the Jensen sarcoma had a greater capacity for asparagine synthesis than either a susceptible line or normal liver. The present studies show that asparaginase resistant lymphomas have higher levels of asparagine synthetase (ASase) than either susceptible tumors or normal tissue. In addition, resistant tumors responded to asparaginase treatment with large increases in ASase levels, a moderate response was noted for normal tissue, and a susceptible tumor gave only a transient response.