Glucose oxidase, uricase, and urease were immobilized on the interior surface of activated polyamide tubing. The shelf-life of such enzyme bearing tubes was at least six months. The tubes were used for continuous-flow analysis of glucose, uric acid, and urea with conventional systems and with hybrid micro-scale systems in which modules of different manufacture were combined. The length of enzyme-bearing tube required for each system was ascertained empirically. Each tube could be used for several thousand assays, but glucose oxidase-bearing tubes were more stable than urease- or uricase-bearing tubes. Results for patients' samples correlated well with results obtained by accepted methods.