The thermal stability at 37 °C of several clinically relevant enzymes and isoenzymes was assessed by measuring changes in enzyme activity as a function of time under incubation and reaction conditions. Selwyn plots were used in the reaction-condition assessments. Except for CK-1 (BB), all the enzymes investigated are stable enough at 37 °C to permit assay. These enzymes were LDH-1, LDH-5, s-AspAT, m-AspAT, apo-s-AspAT, apom-AspAT, ALP-liver, ALP-bone, ALP-intestine, ALT, apo-ALT, CK-2, and CK-3. CK-1 is stable at 37 °C under assay conditions but not under incubation conditions. We specifically avoided using Arrhenius plots to evaluate thermal stability and point out pitfalls inherent in their indiscriminate use.