A continuous-flow clinical analyzer for the routine estimation of urea is described that makes use of an immobilized-enzyme nylon-tube reactor as part of a flow-through system (a Technicon AutoAnalyzer I). Results of blood-urea analyses by use of the immobilized urease are compared with determinations made with the diacetyl monoxime method and the urease solution method. Clinical trials carried out routinely with the immobilized enzyme nylon tube reactor give reliable and reproducible results with high precision and low cost. The reactors are stable to intermittent or continued use for at least four months or for 2000 tests. A method is described in which differential colorimetry is used for determining citrulline in blood and which makes use of the immobilized urease, albeit indirectly.