1. 1.Certain guinea pigs possessed a thermostable l-asparaginase (EC 3.5.1.1) in their sera, while others possessed a thermolabile form of the enzyme. 2. 2.Guinea pig l-asparaginase was stabilized against heat inactivation by Na + and K +. 3. 3.A thermolabile variety of l-asparaginase required a higher concentration of Na + (or K +) for heat stabilization than did a thermostable form of the enzyme. 4. 4.There was no difference in the pH optimum, temperature optimum and electrophoretic mobility between the thermostable and thermolabile l-asparaginase preparations partially purified from each serum sample.