The authors used a differential thermal detector in conjunction with an immobilized urease reactor to determine urea in serum. Samples (120 μl) are introduced into a flow stream and passed through an 'adiabetic' column which is packed with enough insolubilized urease to completely convert urea to ammonia and carbon dioxide. Measured temperature changes are directly proportional to the serum urea concentration. Urea in the presence of protein, bilirubin and hemoglobin can thus be rapidly, simply, and inexpensively measured. Results correlate well with those obtained by the manual diacetyl monoxime and urease/ indophenol methods.